And the attacks may have unpredictable consequences in Iran. The country’s autocratic clerical leadership, which has ruled for nearly half a century since the Islamic Revolution of 1979, has proved its staying power even in the face of multiple domestic uprisings, and proved its resilience under economic sanctions and intense U.S. pressure.

Here are some ways U.S. involvement could play out.

Iran could negotiate

Before Israel launched a surprise attack on June 13, Iran and the United States were discussing limits on Iran’s nuclear program. Iran was rapidly producing fuel close to the levels needed for nuclear weapons, and in exchange for new limits on the program, it would win relief from economic sanctions.

The two sides were nowhere near a final agreement, but signs of a possible compromise had emerged by early June. When Israel attacked Iran, the negotiations collapsed. Still, Iran has signaled that it remains willing to talk, and even a strike on Fordo would not necessarily wipe out prospects of a return to the negotiating table.

But so far, Mr. Trump has not extended many carrots. He called on social media last week for Iran’s “UNCONDITIONAL SURRENDER.”

Iran may lean into nuclear activity

All eyes are on Fordo. But it is possible that Iran has secret nuclear sites aimed at producing weapons that the United States and Israel do not know about, though no public evidence has emerged of such places.

If they do exist, Iran could use whatever it has left to try to accelerate its nuclear program in the wake of an American attack.

With the damage Israeli airstrikes have done to nuclear facilities and the killings of top nuclear scientists, Iran probably lacks the capacity to build a nuclear weapon quickly, analysts said. Still, it could move in that direction and would have fresh incentive to do so.

The war could get bigger and messier

Though Iran has responded to Israeli attacks with missiles and threats of its own, it has refrained from hitting American troops or bases in the Middle East. It has also not struck Arab countries allied with the United States, such as Saudi Arabia or the United Arab Emirates.

Nor has it sent global oil prices soaring by sealing off or harassing traffic in the Strait of Hormuz, a vital oil shipping channel to Iran’s south.

But on Friday, the Iranian foreign minister, Abbas Araghchi, said that if the United States chose to strike Iran, the country reserved the right to retaliate, as it has against Israel. “When there is a war, both sides attack each other. That’s quite understandable. And self-defense is a legitimate right of every country,” he said in an interview with NBC News.

And Iran’s allied militias in the region, including the Houthis in Yemen, Hezbollah in Lebanon and armed groups in Iraq, have not joined the fight. Though many of them have been seriously weakened over the past two years, those Iranian allies could still join the fray.

Talk of regime change

Even if Iran’s supreme leader were to be killed, the religious-military establishment that has tightly held power in Iran for nearly five decades may not fall.

With a war raging, the Islamic Revolutionary Guard Corps, the most powerful branch of Iran’s military, could seize control of the country. They might put in place a more Western-friendly government, or, more likely, replace Ayatollah Ali Khamenei with a more extreme figure who would dig in for a long fight.

If the military does not assert itself quickly, some analysts fear that Iran could plunge into chaos or civil war as different factions struggle for control. But they see little chance for Iran’s liberal opposition, which has been weakened and brutally repressed by the regime, to prevail.