President Trump announced that he would decertify Iran’s compliance with the 2015 nuclear agreement reached with six world powers, including the United States.

Mr. Trump did not immediately pull out of the accord. He essentially deferred to Congress a decision about whether to reimpose sanctions on Iran, which could destroy the agreement. And he added an ultimatum, declaring that if the agreement were not renegotiated to permanently stop Iran from developing nuclear weapons, he would terminate it at some point in the future.

Here are some questions and answers about the agreement and the possible consequences.

Which countries negotiated the agreement and what did it accomplish?

The permanent members of the United Nations Security Council — Britain, China, France, Russia and the United States — plus Germany, known as the P5-plus-1 group, negotiated the agreement with Iran. Disarmament advocates consider it a major achievement of the Obama administration, averting a possible military conflict with Iran and a nuclear arms race in the Middle East.