“Iran will be held fully responsible for lives lost, or damage incurred, at any of our facilities. They will pay a very BIG PRICE! This is not a Warning, it is a Threat,” Mr. Trump tweeted on New Year’s Eve. “Happy New Year!”

Hours into 2020, Iran’s supreme leader responded with a taunt.

“You can’t do anything,” Ayatollah Ali Khamenei wrote on his English-language Twitter account. He added: “If you were logical —which you’re not— you’d see that your crimes in Iraq, Afghanistan… have made nations hate you.”

Two days later, General Suleimani was dead — killed in a drone strike ordered by Mr. Trump.

Nine months of escalation, misjudgments and heated messaging had led to the president’s decision, which stunned both his own military advisers as well as top officials in Tehran.

“It was clear that Iran didn’t expect Trump to retaliate in any meaningful way,” said Mr. Sadjadpour, the Iran expert.

The killing prompted Iran to take a step it had long avoided: a direct and overt strike against the American military. Four days after General Suleimani was killed, Iran fired more than a dozen missiles at two American bases in Iraq. More than 100 American soldiers were injured, but no one was killed, and Mr. Trump and his advisers believed the United States had gotten the better of the exchange.

In the weeks since, they have insisted that their strategy is working, that the steady squeeze of “maximum pressure” will force Iran to yield to their demands. But, at least publicly, Iran remains defiant and wedded to brinkmanship tactics over its nuclear program and regional military influence.

Hours after Iranian missiles landed on the American bases in Iraq, Mr. Khamenei vowed that “harsh revenge” was just beginning. “The United States’ corruptive presence in the region must come to an end,” he told a large crowd in the city of Qum, adding that Iran would not rest until it accomplished that goal.

Mark Mazzetti reported from Washington, Ronen Bergman from Tel Aviv and Farnaz Fassihi from New York. Julian E. Barnes contributed reporting from Washington.