Iran’s clerical rulers have a long history of open animosity toward the United States.

From vowing “Death to America” to striking an American base in Iraq after the U.S. assassination of Iran’s top general, the Iranian government has repeatedly gone to the brink of direct military confrontation with the United States, only to pull back.

Now, with the U.S. strikes on Iranian nuclear facilities, the conflict enters a more dangerous phase.

After the strikes, Abbas Araghchi, the Iranian foreign minister, said on social media that Iran had a legitimate right to respond to U.S. attacks on its nuclear facilities and warned that “Iran reserves all options to defend its sovereignty, interest, and people.”

He accused the United States of violating the U.N. charter and international law, adding, “The events this morning are outrageous and will have everlasting consequences.”

Two senior Iranian officials said in text messages that, before the strikes, there had been hope that Mr. Trump could be dissuaded by those around him who opposed another American war in the Middle East. Mr. Araghchi had been in Turkey for meetings, and his diplomatic outreach to European counterparts, to Arab leaders in the region, and to Turkey, was part of an effort to rally support, according to the two officials.

But it failed. The Atomic Energy Organization of Iran confirmed the U.S. strikes in a statement, saying that around dawn Sunday Iran’s three nuclear sites, Fordo, Natanz and Isfahan, “were attacked in a violent act against international laws, including the Non-Proliferation Treaty, by the enemies of Islamic Iran.”

So far throughout the war with Israel, Iran has refrained from direct attacks on U.S. troops and interests in the Middle East. But Iran’s military commanders have warned that American entry into war would bring retaliation.

The former commander in chief of the Revolutionary Guards Corps, Gen. Mohsen Rezaei, who has a seat at Iran’s Supreme National Security Council, warned on state television hours before the attack that if Mr. Trump entered the war, Iran would strike at American military bases, blow up naval mines in the Persian Gulf and move to close the Strait of Hormuz.

Iran can wreak havoc on the global transit of energy. About 20 million barrels of crude oil and oil products move through the Strait of Hormuz daily.

On Friday, Mr. Araghchi told NBC News that “when there is a war, both sides attack each other. That’s quite understandable.” He added that Iran reserved the right to retaliate against a U.S. attack, as it has against Israel’s. “Self-defense is a legitimate right of every country,” he said.

But Iran’s options are grim. And whatever it does will be a turning point for the Islamic Republic’s nearly five-decade rule.

If it retaliates against the United States, it could face a major war that leads to its collapse or to years of instability, a fate like that of Iraq and Afghanistan.

If it retreats, accepting a cease-fire, it would be a shell of its former self, with its nuclear capacities crippled, its military depleted and little leverage to negotiate for relief from debilitating sanctions.

Iran’s stature in the region, where it has long been viewed as an influential power player, would also be diminished.

“If we do not react, the U.S. will not leave us alone right now when it can so easily come and strike us and leave,” Reza Salehi, a conservative political analyst in Tehran, said in a telephone interview after the attacks. “The big challenge that we face this week is that if we go to the negotiating table, the other side will have more and newer demands, such as our defense abilities, and that will make things complicated.”

The American strikes could also prompt retaliation from Iran’s allied militias in the region. But Israel has weakened Hezbollah in Lebanon and Hamas in Gaza, and Iraqi militias have mostly retreated from attacking American bases.

That leaves the Houthi militia of Yemen, a country sitting along a critical international shipping lane. The Houthis had threatened to break their May truce with Mr. Trump and attack U.S. targets if Washington supported the Israeli attacks on Iran.

“In the event that the Americans become involved in the attack and aggression against Iran alongside the Israeli enemy, the armed forces will target their ships and warships in the Red Sea,” their military spokesman, Yahya Saree, said earlier Saturday.

Before Israel launched its surprise attack on June 13, Iran and the United States had been holding negotiations, mediated by Oman, to curb Iran’s advancing nuclear program in exchange for sanctions relief. The United States demanded that Iran dismantle its program and stop enriching uranium. The U.S. had also proposed that Iran enter into a nuclear consortium with Arab countries for access to civilian-grade nuclear fuel.

Iran was preparing a response, but officials had said that giving up enrichment of uranium on Iran’s soil was a red line, and they would not dismantle the program.

Those talks collapsed after the Israeli attack, two days before Iran and the United States were scheduled to meet in Oman.