The United States and Iran on Monday missed their 60-day deadline to reach a deal to curtail Iran’s nuclear program under the terms of the cease-fire they agreed to in June, underlining the breakdown of a truce that President Trump promised would end the war.

Mr. Trump had announced the so-called memorandum of understanding between the two countries with great fanfare two months ago. But the short-lived agreement failed to reopen the Strait of Hormuz, which Iran effectively blockaded after Israel and the United States attacked in late February, or end a conflict that has entered its sixth month.

The expiration of the deadline is largely symbolic, as both Mr. Trump and Iran’s leaders have said in recent weeks that the deal is basically dead. The original deal also provided for the 60-day period to be extended if necessary, which would have been likely given the sensitive and complex nature of the negotiations.

But the milestone nonetheless shows how Mr. Trump has struggled to achieve his war aims — like forcing Iran to curtail its nuclear ambitions — and how emboldened the Iranian government feels, with the two countries at risk of further military conflict.

Esmail Baghaei, the Iranian foreign ministry spokesman, told reporters on Monday that the negotiations on the nuclear deal “never began,” adding that the 60-day period was “fundamentally no longer relevant.”

There have been no publicly reported U.S. strikes on Iranian targets since late July. But the United States and Iran appear to be gearing up for a protracted campaign of economic attrition that could easily tip over into renewed warfare, analysts said. Iran is wielding its control of the Strait of Hormuz, a critical waterway for oil and gas shipping, to choke global energy markets, raising pressure on Mr. Trump to reach a deal.