Ask diplomats who have negotiated with Iran, and they usually describe it with some variant of: Brace yourself, it takes a long time.

It took the better part of two years to put together the Obama-era agreement that all but halted Iran’s nuclear program. After President Trump scrapped that deal in his first term, it took 15 months for the Biden administration to negotiate a way to piece it back together — at which point Iran’s supreme leader, Ayatollah Ali Khamenei, vetoed the near-final agreement. So what could Mr. Trump, dangling the possibility that last-minute diplomacy could provide an alternative to bombing Iran’s main uranium enrichment facility, hope to accomplish in the two-week window he has given himself to make a decision?

Not much, the veterans of such negotiations warn. But then again, the environment is very different this time.

Ayatollah Khamenei is the final word in all foreign policy issues — but he is also most likely in hiding, American intelligence officials say.

Iran’s foreign minister and lead negotiator, Abbas Araghchi, says he is open to placing limitations on Iran’s nuclear output similar to what he and his colleagues negotiated with the United States a decade ago.

But on Friday, he told his European counterparts in Geneva that Iran would never negotiate as long as Israel was dropping missiles on its military bases and nuclear facilities, and carrying out targeted killings of Islamic Revolutionary Guards Corps officers and nuclear scientists.