Over the weekend, President Trump rejected a return to full-scale war. But as he tries to find a way out of a conflict he started, Iran has proved a stubborn enemy.

On Tuesday evening Iran fired multiple missiles on American bases in Jordan. The U.S. military intercepted the missiles with apparent ease, but the incident has drawn Mr. Trump once again back into fighting.

“We’re going to be hitting them very hard, because it’s our turn to hit them,” Mr. Trump told reporters in the Oval Office on Wednesday.

A senior Pentagon official said U.S. forces would hit back at a time and place of their choosing. All indications were that the timing was imminent.

The Iranian move — and Mr. Trump’s threat — comes as the war has widened quickly, with Saudi Arabia joining U.S. forces in striking Iranian proxies in Iraq, an American tanker in Egypt set ablaze from an apparent drone strike and tensions rising between Ukraine and Iran.

The war, which Mr. Trump has been trying to bring to a diplomatic conclusion since April, is now threatening to grow bigger at the very moment the United States wants to move past it.

“When Trump started the war, he thought he could control it,” said Vali Nasr, a professor at the Johns Hopkins School of Advanced International Studies. “The Iranian response is to extend the battlefield much, much wider, so the United States has to defend a broader battlefield.”

Experts said that at this stage in the war, which the United States launched with Israel five months ago, Iran wants to demonstrate the military adage that the enemy always gets a vote.

“Iran wanted to demonstrate that they are not going to sit back and wait for the U.S. or Israel to strike at a time of their preference,” said Jason Campbell of the Middle East Institute. “They are trying to seize the initiative, or at the very least keep the U.S. and Israel on their back foot.”

U.S. military commanders prepared a plan for Mr. Trump that would have started another days-long bombing campaign. U.S. forces had moved air assets to Israel in preparation for those attacks, and those planes remain in place should Mr. Trump change his mind in the face of the Iranian action.

In the deliberations over that plan, Gen. Dan Caine, the chairman of the Joint Chiefs of Staff, cautioned privately that resuming major combat operations against Iran was doable but would dangerously deplete interceptors available to the military’s Central Command, according to two people briefed on the discussions.

Adm. Brad Cooper, the head of Central Command, said that if Mr. Trump did not resume major combat operations, the United States should stop the bombing campaign around the Strait of Hormuz because it has reached the limit of its effectiveness, the two officials said, speaking on the condition of anonymity to discuss operational matters.

The U.S. strikes in the strait are aimed at reopening the international waterway, which the conflict effectively closed. As a result, oil prices have increased overall since the start of the war.

A spokesman for Admiral Cooper declined to comment citing the confidential discussions.

In another Fox interview on Tuesday, Mr. Trump seemed to weigh the misery and humanitarian costs that resuming major operations, including more strikes against Iran’s infrastructure, including bridges and power plants, would inflict on Iran’s civilian population.

“And who am I hurting there?” he said. “I’m hurting the people. So I’m not looking to do that.”

But between Mr. Trump’s doubts and his vow to hit Iran hard, the U.S. strategy is not at all clear.

His threat aside, Mr. Trump seems more focused on bringing Iran back to the negotiating table. Some of the countries that have been mediating talks, including Pakistan and Qatar, are trying to do the preparatory work to restart such discussions.

Experts, however, are deeply skeptical about what kind of deal Mr. Trump can make.

Officials in the Trump administration now rarely discuss the list of priorities they had at the beginning of the war, which included eliminating Iran’s nuclear program but also eroding Tehran’s missile capability and curbing its proxy forces.

“The justification for the war has fluctuated considerably over the last few months,” Mr. Campbell said. “It has been months since you have heard Trump or any other senior official mention anything other than the nuclear program.”

For Mr. Trump, the single most important goal is reopening the Strait of Hormuz, and to do it in a way that does not look like a loss for the United States.

But unless traffic can begin to flow through the waterway, without being cut off by an Iranian attack on a tanker or insurance companies’ doubts about safety, oil prices will rise, not recede.

“As long as issues such as who controls the Strait of Hormuz remain unresolved, let alone concerns regarding Iran’s nuclear ambitions and other initial drivers of the war, the cycle of hostilities and temporary pauses will continue,” said Mona Yacoubian, director of the Middle East Program at the Center for Strategic and International Studies.

In an odd way, the Trump administration’s current struggles with Iran are at least partly the result of the U.S. success at the start of the war.

Ayatollah Ali Khamenei, Iran’s supreme leader for nearly 37 years, was killed in the opening strike by an Israeli attack based on American intelligence. He was quickly succeeded by his son, Mojtaba Khamenei, a far more hard-line leader who does not share his father’s caution. U.S. intelligence agencies have assessed the son is more open to pursuing a nuclear weapon than his father, who had long renounced their use.

What is more, the leaders of the old regime — most of whom were also killed in the war — often choreographed retaliatory strikes in a way meant to save face at home while not escalating hostilities against the U.S.

“That was the old Khamenei,” Mr. Nasr said.

Iran’s new leaders, he added, want to inflict as much damage on the United States and its allies as it can to deter a future war against their country.

Nicholas Hopton, a former British ambassador to Iran, Qatar and Yemen, called the five-month conflict “a misadventure” that is not winnable by military means alone.

“The president of the United States still doesn’t seem to have made up his mind whether he wants a negotiated solution to this or to try for some kind of military conclusion, which all observers who know the situation in Iran and have worked in the Gulf would say is highly unlikely to work,” Mr. Hopton, who is now a distinguished fellow with the Royal United Services Institute for Defense and Security Studies in London, said in webinar on Wednesday.

Jonathan Swan, Erica L. Green, Edward Wong and Farnaz Fassihi contributed reporting.