Iran built Fordo deep inside a mountain to withstand aerial attacks.

“All planes are now outside of Iran air space,” he said in a post on social media, adding that a “full payload” of bombs had been dropped on Fordo, the heavily fortified underground facility in Iran that is critical to its nuclear program. “All planes are safely on their way home.”

The three sites that Mr. Trump said were hit included Iran’s two major uranium enrichment centers: the mountain facility at Fordo and a larger enrichment plant at Natanz, which Israel struck several days ago with smaller weapons. The third site, near the ancient city of Isfahan, is where Iran is believed to keep its near-bomb-grade enriched uranium, which inspectors saw just two weeks ago.

Three senior Iranian officials, speaking on condition of anonymity because they were not authorized to speak publicly, said that they believed American forces had bombed Fordo and Natanz at around 2.30 a.m. on Sunday in Iran.

After a week of mixed signals, President Trump, who has long vowed to steer America clear of overseas “forever wars,” authorized U.S. forces to strike Iran’s most heavily fortified nuclear installation, deep underground. The goal, American and Israeli officials have said, is to prevent Iran from building a nuclear bomb.

For days, Mr. Trump had been weighing whether to use the powerful munitions needed to destroy Iran’s deeply buried nuclear enrichment facilities, at an installation known as Fordo. Only American bombs known as bunker busters are believed up to the job, and only American aircraft can deliver them.

Israel and Iran, sworn enemies for decades, have been exchanging attacks since June 13, when the Israelis launched a surprise assault that targeted Iranian infrastructure, including nuclear installations, and military leaders. Prime Minister Benjamin Netanyahu said his nation had no choice but to act if it wanted to stave off a nuclear “holocaust.”

Iran responded with missile barrages of its own, as well as offers to resume negotiations over its nuclear development program.

Just days ago, the Trump administration appeared intent on distancing itself from the conflict. “We are not involved in strikes against Iran and our top priority is protecting American forces in the region,” Secretary of State Marco Rubio declared.

But Mr. Trump, when he was not urging peace talks, began sounding increasingly belligerent.

On Tuesday, he went so far as to make a direct threat against Iran’s supreme leader, Ayatollah Ali Khamenei, saying that “we know exactly where” he is and calling him “an easy target.” He said, “We are not going to take him out (kill!), at least for now.” But he warned, “Our patience is growing thin.”

Mr. Trump called for Iran’s “complete surrender.”

This week, when asked about assessments by U.S. intelligence agencies and his director of national intelligence, Tulsi Gabbard, that Iran was not actively working toward a nuclear weapon, Mr. Trump said flatly that they were wrong. Iran, he insisted, was months — if not weeks — away from being able to produce a bomb.

Here is what else to know:

• What’s next? Now that Mr. Trump has sent American bombers to help Israel destroy a uranium enrichment facility in Iran, it will most likely initiate a more dangerous phase in the war. Here are some ways that could play out, and a look at how the U.S. military’s powerful bunker-busting bombs work.

• Saturday strikes: Israel launched a wave of airstrikes against missile sites, a nuclear facility and munitions storage sites in Iran, while Iran fired a barrage of ballistic missiles and launched drones into Israel. The southern region that Israel targeted in Iran would likely have been on any potential flight path used by U.S. warplanes on the way to strike Fordo.

• Commanders killed: Israel’s military said it killed Mohammed Said Izadi, Behnam Shahriyari and Aminpour Joudaki, commanders from the Islamic Revolutionary Guard Corps. Mr. Izadi and Mr. Shahriyari were both senior officials in the Quds Force, which oversees and supports proxy militias around the Middle East, according to Israel’s defense ministry. The deaths were not immediately confirmed by Iran.

• Evacuations: The U.S. Department of State has begun evacuating Americans from Israel, said the American ambassador, Mike Huckabee. In a post on social media, he encouraged Americans in Israel and the West Bank to fill out a form requesting evacuation, which could be by cruise ship, commercial flight, charter flight or a flight operated by the U.S. government.

Iran, which is a signatory to the Nuclear Nonproliferation Treaty, has maintained that its nuclear program is for peaceful purposes.

Iran built the centrifuge facility at Fordo in the 2000s, knowing that it needed to bury it deep to prevent it from being attacked. In 1981, using F-15 and F-16 fighter jets, Israel bombed a nuclear facility near Baghdad as part of its effort to stop Iraq from acquiring nuclear weapons. That facility was above ground.

“The Iranians fully understood that the Israelis would try to get inside their programs and they built Fordo inside of a mountain a long time ago to take care of the post-Iraq problem” presented by the 1981 strike, said Vali Nasr, an Iran expert who is a professor at Johns Hopkins University.

Over the years, the Israelis drew up a variety of plans to attack Fordo in the absence of U.S.-supplied bunker buster bombs. Under one of those plans, which they presented to senior officials in the Obama administration, Israeli helicopters loaded with commandos would fly to the site. The commandos would then fight their way inside the facility, rig it with explosives and blow it up, former U.S. officials said.

Israel successfully mounted a similar operation in Syria last year when it destroyed a Hezbollah missile production facility.

But Fordo would have been a much more dangerous endeavor, military officials said.

“The Israelis have sprung a lot of clandestine operations lately, but the physics of the problem remain the same,” said Gen. Kenneth F. McKenzie Jr., who was in charge of the Iran war plans when he ran the Pentagon’s Central Command after General Votel. “It remains a very difficult target.”

It remains unclear how much damage the U.S. strikes early Sunday may have done to Fordo. Three senior Iranian officials, speaking on condition of anonymity because they were not authorized to speak publicly, said that Fordo was hit by bombs around 2:30 a.m.

“It feels like we’re in a dark cave,” said Arta, an Iranian who fled Tehran on Tuesday and was able to briefly send a few messages over Instagram late Friday.

Like many others who have exchanged messages with The New York Times over the last week, he asked to be identified only by his first name to avoid scrutiny by the authorities.

“Even SMS texts don’t go through sometimes,” he said.

Many Iranians rely on virtual private networks, or VPNs, to evade government restrictions on the internet, but many of those services have been disrupted since Israel’s attacks began. On Saturday, as some connection returned, providers urged their users to act cautiously.

“For your own sake, don’t spread the link, the server will disconnect, and our work will only get harder,” one organizer wrote on a VPN provider’s Telegram channel. The organizer warned that reports of disconnection were increasing again, and asked subscribers to not share their product link because their server was overwhelmed.

Since at least Wednesday, the Iranian government has significantly restricted internet access across the country, with a government spokeswoman, Fatemeh Mohajerani, saying on Friday that the measures were taken because of “cyberattacks and security” reasons.

For Iranians abroad with loved ones in the country, the blackout has multiplied anxieties that were already high as news reports about Israeli evacuation orders and airstrikes rolled in.

Incoming international calls have also been blocked, forcing people in Iran to call their family and friends abroad directly. But making international calls from Iran means very high fees, which many in the country can’t afford.

Still, Iranians have sought creative ways around the restrictions.

On Thursday, during the height of the blackout, a group of Iranians managed to get online and speak with people outside the country through Clubhouse, an audio app that is popular in Iran. At one point, nearly 1,700 Iranians joined the call, hoping for help in reaching their loved ones.

For hours, Iranians abroad took turns sharing the names and numbers of their friends and relatives so that people inside Iran could connect them through Clubhouse.

“Dad? Can you hear me? Do you have insulin?” asked a woman who managed to get a hold of her elderly father when the organizers on Clubhouse dialed his number. “I went and bought it, don’t worry,” her father tried to reassure her.

“When you speak to Sanaz, tell her happy birthday for us,” another woman told her niece, who was in Canada. “Don’t cry, don’t worry about us,” the woman said, echoing what many Iranians in the country kept repeating to their nervous family members.

As Iranians abroad have tried to reach relatives, those inside the country have made a show of public solidarity since Israel’s attacks began last week.

Hotels and hostels have advertised free shelter. People in line for bread at a bakery shared what little of it remained, and others said they were feeding the stray cats still wandering Tehran. A father and daughter handed out drinks to people waiting in a long line for gas. Others delivered water to those stranded on the roads after their GPS-guided maps stopped working in the blackout.

A woman named Niloofar, who lives in a residential district of Tehran that Israel warned should be evacuated on Monday, said that many Iranians, despite their fears, had also expressed a determination to keep living as they always had.

“My sister and I tried so hard to get our parents to evacuate,” she said in a message on Telegram. But her mother was cooking olovieh, a classic Persian dish, “while my dad watched soccer,” she added. “At the end, they both refused to leave.”

The B-2 flights were initially tracked on social media before 1 a.m. Eastern time on Saturday. Some flight trackers said on social media that the destination of the aircraft is Guam, the U.S. territory, which has several military installations. The bombers appeared to be accompanied by refueling tankers for portions of the journey, the flight tracking data showed.

Additional Air Force F-22, F-16 and F-35 fighter jets have crossed Europe and are now at bases in the Middle East, or are arriving there, a U.S. official said on Saturday. The jets could escort B-2 bombers that target Fordo, or protect U.S. bases and troops in the region in the event of Iranian retaliatory strikes.

Moving planes does not mean a final decision has been made about whether to strike. It is not unusual to shift military assets into position to provide options to the president and military commanders even if they are not ultimately deployed.

Mr. Trump left his golf club in Bedminster, N.J., and returned to the White House to confer with his national security team, which he was also scheduled to meet with on Sunday. Mr. Trump typically spends both weekend days out of town at one of his properties.

A White House spokeswoman declined to comment.

Mr. Trump has made clear he is weighing whether to have the United States join Israel’s effort to curtail Iran’s ability to acquire a nuclear weapon, a line he has drawn repeatedly over the years.

But he also gave himself extra time to say what he intends to do. Karoline Leavitt, the White House press secretary, told reporters on Wednesday that the president would make a decision within the next two weeks as he gives Iran another chance to engage in talks.

The president has been seeking a deal with Iran for months, but he become frustrated at the refusal of Iranian officials to agree to a proposal to end uranium enrichment on Iranian soil. At the same time, the U.S. intelligence community came to the conclusion in early June that Prime Minister Benjamin Netanyahu of Israel planned to move forward with strikes against Iran, with or without U.S. help.

Those strikes began on June 12 and have continued since, killing multiple members of Iran’s military leadership and drawing retaliatory strikes from Iran against Israel.

Mr. Trump has been torn between the opportunity to carry out what could be a devastating blow against Iran’s nuclear facilities at a moment when Iran’s defenses have been greatly weakened and the concern that doing so would risk the kind of protracted U.S. military engagement in the region that he campaigned against in 2016 and 2024. That debate has also split his supporters.

On Friday, Mr. Trump reiterated his time frame for a decision on military action “within two weeks,” saying the thinking behind it was “just time to see whether or not people come to their senses.”

As questions mounted about whether Mr. Trump was preparing to enter the war, Israel stepped up attacks on nuclear sites that are part of the supply chain Iran has built up over the past two decades, enabling it to enrich uranium.

For the second time in eight days, Israel focused on sites in the mountains near the ancient capital of Isfahan. The International Atomic Energy Agency, the U.N. body that monitors nuclear production sites, reported that a “centrifuge manufacturing workshop” had been targeted. That is one of the workshops where Iran produces the machines that spin at supersonic speeds to enrich uranium.

The same kind of machinery sits under the mountain at Fordo. The agency’s inspectors often visited the workshop, and I.A.E.A. monitoring cameras were installed there.

“We know this facility well,” said Rafael Mariano Grossi, the director general of the agency. “There was no nuclear material at this site, and therefore the attack on it will have no radiological consequences.”

But by hitting the workshops, Israel is clearly seeking to impede Iran from rebuilding nuclear enrichment sites elsewhere, presumably in secret, if the Fordo plant is disabled or destroyed.

Still unknown, however, is whether Iranian scientists, many of whom have been killed in the past week, have replicated the Isfahan workshop in undeclared sites elsewhere in the country.

On Wednesday, the nuclear agency reported that Israel had also attacked the Tehran Research Center, where the most delicate and complex parts of the centrifuges — the fast-spinning rotors — are produced and tested.

Mr. Izadi was one of the few people who knew in advance about Hamas’s plan to launch the surprise attack, The New York Times reported last year.

Israeli officials said Mr. Izadi led the Palestinian affairs branch in the Quds Force, the arm of Iran’s powerful Islamic Revolutionary Guards Corps responsible for foreign operations. Israel said it had struck an apartment where Mr. Izadi was staying in central Iran overnight between Friday and Saturday.

There was no immediate comment from Iran.

A second commander Israel said it killed was identified as Behnam Shahriyari, another Quds Force commander who oversaw weapons transfers to Iran-backed paramilitary forces in the region.

Mr. Shahriyari had been sanctioned by the U.S. government since 2011 on accusations of transferring weapons on behalf of the Revolutionary Guards to Hezbollah, the Lebanese militant group.

Effie Defrin, a spokesman for the Israeli military, accused Mr. Shahriyari of being behind financial transfers to allied militant groups in the region.

“Izadi and Shahriyari were at the forefront of Iran’s project to export war into Israeli territory,” Mr. Defrin said in a statement. “In this operation, we have brought the war to them — we eliminated them inside Iran.”

Iran has long backed a network of militias across the Middle East in an attempt to extend its power and influence across the region and menace its enemy, Israel. They include Hamas, Hezbollah and the Houthis, who control parts of Yemen.

In 2019, the United States imposed sanctions on Mr. Izadi, saying he had provided millions of dollars to Hamas. Britain did the same four years later to counter what it called “unprecedented threats from the Iranian regime.”

The Times reported that in August 2023, Khalil al-Hayya, a senior Hamas leader in Gaza, said in a closed-door meeting of the group’s leadership that he had spoken to Mr. Izadi the previous month to outline its plan to launch a huge assault on Israel, according to internal minutes of the group’s leadership meetings.

Mr. al-Hayya said he had told Mr. Izadi that Hamas would need help with striking sensitive sites during “the first hour” of the attack. According to the documents, Mr. Izadi said that Hezbollah and Iran welcomed the plan in principle, but that they needed time “to prepare the environment.”

After the war in Gaza began, Mr. Izadi remained in direct contact with Hamas’s top leadership and sought to aid them by transferring equipment and funds into the territory, according to two Israeli defense officials. They spoke on the condition of anonymity because they were not authorized to speak publicly.

Last year, as Israeli attacks on Hezbollah in Lebanon intensified, Mr. Izadi left his longtime base in that country for Iran.

He eventually wound up in a safe house belonging to the Revolutionary Guards in the Iranian city of Qum, the two officials said — the same apartment where he was killed overnight.

Israel’s military on Saturday also released video of a missile strike on a car, which it said showed the Israeli strike that killed Mr. Shahriyari as his vehicle drove through western Iran.

Mr. Defrin said Israeli forces had also killed a Revolutionary Guard commander of drone operations identified as Aminpour Joudaki. He said Mr. Joudaki had coordinated strikes on Israel launched from southwestern Iran.

“We are constantly afraid, and the psychological toll has been heavy,” said Rawan Muhaidat, 28, a mother of two in the town of Kafr Asad in northern Jordan.

The sight of Iranian missiles overhead, and the booms of air defenses shooting them down before they reach Israel, have terrified her children, who cower between her and her husband as they worry that their home could be struck.

“Every time a rocket passes and explodes, we think, ‘This is the one,’” Ms. Muhaidat said.

Adding to many people’s fears is the possibility that President Trump will grant Israel’s request that the United States intervene by dropping 30,000-pound bombs on an Iranian nuclear enrichment facility buried deep underground.

Such a move, experts say, could push Iran to retaliate against American military bases or allies across the Middle East, or to activate proxy forces, like the Houthis in Yemen, to snarl trade routes or damage oil infrastructure, harming the global economy.

“We’re opening a Pandora’s box,” said Narges Bajoghli, an associate professor of Middle East studies at Johns Hopkins University. “Iran is not going to raise the white flag of surrender.”

The war highlights how significantly the power structure across the Middle East has shifted in recent years.

Just over a half-decade ago, Israel largely focused on its conflict with the Palestinians while waging a shadow war with Iran through occasional assassinations and other covert attacks. But it avoided direct confrontation, partly for fear of retaliation from the network of militias that Iran supported in Lebanon, Syria, Iraq and Yemen.

At that time, most Arab countries shunned Israel, a Jewish-majority democracy, for its treatment of the Palestinians, and many resented the predominantly Persian Iran for what they considered its destructive meddling in the Arab world. But a few Arab states began to see Israel as a potential partner in dealing with their own concerns about Iran and established formal diplomatic relations.

That picture has now changed.

The deadly surprise attack by the Palestinian militant group Hamas in October 2023 heightened Israel’s sense of vulnerability, and the country has become increasingly aggressive in striking out against perceived threats far beyond its borders.

For Iran, the wars in Gaza and Lebanon, and the ouster last year of President Bashar al-Assad of Syria, decimated its regional proxy network and left it even more isolated.

Powerful Arab countries, including Saudi Arabia, the United Arab Emirates and Egypt, pursued their own diplomatic tracks with Tehran to decrease tensions. Now, they also hope to avoid a war in their neighborhood that could put them in the cross hairs because of their partnerships with the United States.

The current conflict began on June 13, at a bad time for the international institutions that were established to try to contain such hostilities.

Israel’s war in Gaza, which began after Hamas’s Oct. 7, 2023, attack, has killed more than 50,000 people and caused widespread destruction and hunger in Gaza.

Few seem to expect that the warring parties in the new conflict will be held accountable for killing civilians or striking hospitals, as Israel has done repeatedly in Gaza — sometimes because Hamas has built tunnels beneath them — and as Iran did in Israel on Thursday.

Expectations are low that action by the United Nations Security Council will stop the war, not least because the United States would almost certainly veto any measure that called for its end. And Iran’s foreign minister, Abbas Araghchi, told senior European officials during talks in Geneva on Friday that Iran would not negotiate under fire.

Mr. Trump dismissed the European efforts anyway, saying: “Iran doesn’t want to speak to Europe. They want to speak to us.”

He has said he will decide “within the next two weeks” whether the United States will bomb Iran.

That lack of international action to stop the war has left Mr. Trump and Prime Minister Benjamin Netanyahu of Israel free to proceed as they choose, Professor Bajoghli said.

“We’re entering a new international era, a new world order, and it seems to in some ways be an old world order of force and the law of the jungle, but with 21st-century technology and weaponry,” she said.

Israel initiated the war with a multipronged surprise attack that damaged Iranian military and nuclear sites, largely destroyed air defenses, and killed top nuclear scientists and military officials in their homes, as well as a number of civilians. Iran has responded by firing barrages of ballistic missiles at Israel, some of which have struck civilian apartment towers. At least 224 people have been killed in Iran and 24 in Israel.

Mr. Netanyahu has said that Israel launched the attack to prevent Iran from building a nuclear weapon, which Israel would consider an existential threat. He has also suggested the more expansive goals of regional transformation and regime change.

“We are changing the face of the Middle East, and that can lead to radical changes inside Iran itself,” he said on Monday.

Iran insists its nuclear program is peaceful, and United States intelligence agencies have assessed that Iran has not decided to seek a nuclear weapon, although that could change if the United States bombs Iran’s underground enrichment facility in Fordo or if Israel kills Iran’s supreme leader, Ayatollah Ali Khamenei.

For his part, Ayatollah Khamenei has threatened to retaliate if the United States strikes Iran.

“The harm the U.S. will suffer will definitely be irreparable if they enter this conflict militarily,” he said in a televised address on Wednesday.

The war is hugely unwelcome in the rest of the Middle East, where other governments would prefer to put the region’s conflicts behind them so they can rebuild what has been destroyed and focus on strengthening their economies.

There is little affinity for either of the warring parties. Most Arab states shun Israel, and even governments that have established diplomatic relations with it have condemned how it has fought in Gaza and its attack on Iran.

But that does not mean they support Iran. In a predominantly Sunni Muslim region, most Arab governments see Iran’s revolutionary Shiite theocracy as anathema, and many people in Yemen, Syria, Lebanon and elsewhere resent Iran’s interventions in their countries.

Many Middle East leaders have complicated reactions to the war, said Dina Esfandiary, the lead Middle East analyst at Bloomberg Economics, a research group.

“Officials in the region are quietly glad that Iran’s top brass is being taken out bit by bit, that Iranian proxies and their leaderships are being taken out bit by bit,” she said. “That, from their perspective, gets rid of one of the real threats in the region for them.”

But many also fear an expanded role in the Middle East for Israel, she added, given the tremendous military and diplomatic support it receives from the United States.

That leaves other countries wondering, she said, “Where is Israel going to go next?”

Rana F. Sweis contributed reporting from Amman, Jordan, and Falih Hassan from Baghdad.

And in a remarkable move, the officials add, Ayatollah Khamenei has even named three senior clerics as candidates to succeed him should he be killed, as well — perhaps the most telling illustration of the precarious moment he and his three-decade rule are facing.

Ayatollah Khamenei has taken an extraordinary series of steps to preserve the Islamic Republic ever since Israel launched a series of surprise attacks last Friday.

Though only a week old, the Israeli strikes are the biggest military assault on Iran since its war with Iraq in the 1980s, and the effect on the nation’s capital, Tehran, has been particularly fierce. In only a few days, the Israeli attacks have been more intense and have caused more damage in Tehran than Saddam Hussein did in his entire eight-year war against Iran.

Iran appears to have overcome its initial shock, reorganizing enough to launch daily counterstrikes of its own on Israel, hitting a hospital, the Haifa oil refinery, religious buildings and homes.

Iran’s top officials are also quietly making preparations for a wide range of outcomes as the war intensifies and as President Trump considers whether to enter the fight, according to the Iranian officials, who insisted on anonymity because they were not authorized to speak publicly about the ayatollah’s plans.

Peering inside Iran’s closely guarded leadership can be difficult, but its chain of command still seems to be functioning, despite being hit hard, and there are no obvious signs of dissent in the political ranks, according to the officials and to diplomats in Iran.

Ayatollah Khamenei, 86, is aware that either Israel or the United States could try to assassinate him, an end he would view as martyrdom, the officials said. Given the possibility, the ayatollah has made the unusual decision to instruct his nation’s Assembly of Experts, the clerical body responsible for appointing the supreme leader, to choose his successor swiftly from the three names he has provided.

Normally, the process of appointing a new supreme leader could take months, with clerics picking and choosing from their own lists of names. But with the nation now at war, the officials said, the ayatollah wants to ensure a quick, orderly transition and to preserve his legacy.

“The top priority is the preservation of the state,” said Vali Nasr, an Iran expert and professor of international affairs at Johns Hopkins University. “It is all calculative and pragmatic.”

Succession has long been an exceedingly delicate and thorny topic, seldom discussed publicly beyond speculations and rumors in political and religious circles. The supreme leader has enormous powers: He is the commander in chief of the Iran Armed Forces, as well as the head of the judiciary, the legislature and the executive branch. He is also a Vali Faqih, meaning the most senior guardian of the Shiite faith.

Ayatollah Khamenei’s son Mojtaba, also a cleric and close to the Islamic Revolutionary Guards Corps, who was rumored to be a front-runner, is not among the candidates, the officials said. Iran’s former conservative president, Ibrahim Raisi, was also considered a front-runner before he was killed in a helicopter crash in 2024.

Since the war started, Ayatollah Khamenei has delivered to the public two recorded video messages, against a backdrop of brown curtains and next to the Iranian flag. “The people of Iran will stand against a forced war,” he said, vowing not to surrender.

In normal times, Ayatollah Khamenei lives and works in a highly secure compound in central Tehran called the “beit rahbari” — or leader’s house — and he seldom leaves the premises, except for special occasions like delivering a sermon. Senior officials and military commanders come to him for weekly meetings, and speeches for the public are staged from the compound.

His retreat to a bunker shows how furiously Tehran has been struck in a war with Israel that Iranian officials say is unfolding on two fronts.

One is being waged from the air, with Israeli airstrikes on military bases, nuclear facilities, critical energy infrastructure, commanders and nuclear scientists in their apartment buildings in tightly packed residential neighborhoods. Some of Iran’s top commanders were summarily wiped out.

Hundreds of people have also been killed and thousands of others injured, with civilians slain across Iran, human rights groups inside and outside the country say.

But Iranian officials say that they are fighting on a second front, as well, with covert Israeli operatives and collaborators scattered on the ground across Iran’s vast terrain, launching drones at critical energy and military structures. The fear of Israeli infiltration among the top ranks of Iran’s security and intelligence apparatus has rattled the Iranian power structure, even Ayatollah Khamenei, officials say.

“It is clear that we had a massive security and intelligence breach; there is no denying this,” said Mahdi Mohammadi, a senior adviser to Iran’s speaker of Parliament, Gen. Mohammad Ghalibaf, in an audio recording analyzing the war. “Our senior commanders were all assassinated within one hour.”

Iran’s “biggest failure was not discovering” the months of planning Israeli operatives had conducted to bring missiles and drone parts into the country to prepare for the attack, he added.

The country’s leadership has been preoccupied with three central concerns, officials say: an assassination attempt against Ayatollah Khamenei; the United States’ entering the war; and more debilitating attacks against Iran’s critical infrastructure, like power plants, oil and gas refineries and dams.

Should the United States join the fight, the stakes would multiply significantly. Israel says that it wants to destroy Iran’s nuclear program, but experts say that only the United States has the bomber — and the enormous 30,000-pound bomb — that might be capable of penetrating the mountain where Iran has built its most critical nuclear enrichment facilities, Fordo.

Iran has threatened to retaliate by attacking American targets in the region, but that would only risk a wider, and possibly more devastating, conflict for Iran and its adversaries.

The fear of assassination and infiltration within Iran’s ranks is so widespread that the Ministry of Intelligence announced a series of security protocols, telling officials to stop using cellphones or any electronic devices to communicate. It has also ordered all senior government officials and military commanders to remain below ground, according to two Iranian officials.

Almost every day, the Ministry of Intelligence or the Armed Forces issue directives for the public to report suspicious individuals and vehicle movements, and to refrain from taking photographs and videos of attacks on sensitive sites.

The country has also been in a communication blackout with the outside world. The internet has been nearly shut down, and incoming international calls have been blocked. The Ministry of Telecommunications said in a statement that these measures were to find enemy operatives on the ground and to disable their ability to launch attacks.

“The security apparatus has concluded that, in this critical time, the internet is being abused to harm the lives and livelihoods of civilians,” said Ali Ahmadinia, the communications director for President Masoud Pezeshkian. “We are safeguarding the security of our country by shutting down the internet.”

On Friday, the Supreme National Security Council took it a step further, announcing that anyone working with the enemy must turn themselves into the authorities by the end of the day on Sunday, hand over their military equipment and “return to the arms of the people.” It warned that anyone discovered to be working with the enemy after Sunday would face execution.

Tehran has largely emptied out after orders by Israel to evacuate several highly populated districts. Videos of the city show highways and desolate streets that are typically clogged with bumper-to-bumper traffic. In interviews, residents of Tehran who remained in the city said security forces had set up checkpoints on every highway, on smaller roads and at entry points in and out of the city to conduct ad hoc searches.

Mohammad Ali Abtahi, a reformist politician and a former vice president, said in a telephone interview from Tehran that Israel had miscalculated Iranians’ reaction to the war. Mr. Abtahi said that the deep political factions that are typically in sharp disagreement with one another had rallied behind the supreme leader and focused the country on defending itself from an external threat.

The war has “softened the divisions we had, both among each other and with the general public,” Mr. Abtahi said.

Israel’s attacks have set off a resurgence of nationalism among many Iranians, inside and outside the country, including many critical of the government. That sense of common cause has emerged in a torrent of social media posts and statements by prominent human rights and political activists, physicians, national athletes, artists and celebrities. “Like family, we may not always agree but Iran’s soil is our red line,” wrote Saeid Ezzatollahi, a player with Iran’s national soccer squad, Team Melli, on social media.

Hotels, guesthouses and wedding halls have opened their doors free of charge to shelter displaced people fleeing Tehran, according to Iranian news media and videos on social media. Psychologists are offering free virtual therapy sessions in posts on their social media pages. Supermarkets are giving discounts, and at bakeries, customers are limiting their own purchases of fresh bread to one loaf so that everyone standing in line can have bread, according to videos shared on social media. Volunteers are offering services, like running errands to checking on disabled and older residents.

“We are seeing a beautiful unity among our people,” said Reza, 42, a businessman, in a telephone interview near the Caspian Sea, where he is taking shelter with his family. Using only one name to avoid scrutiny by the government, he added: “It’s hard to explain the mood. We are scared, but we are also giving each other solidarity, love and kindness. We are in it together. This is an attack on our country, on Iran.”

Narges Mohammadi, a Nobel Peace Prize winner and the country’s most prominent human rights activist, has spent decades in and out of jail, pushing for democratic change in Iran. But even she warned against the attacks on her country, telling the BBC this past week that “Democracy cannot come through violence and war.”