President Trump has vowed to secure a “far better” agreement with Iran than one struck by President Barack Obama more than a decade ago.

That agreement, commonly known as the 2015 Iran nuclear deal, was designed to prevent Iran from acquiring a nuclear weapon. It required Iran to limit its nuclear program in return for economic sanctions relief.

Mr. Trump withdrew from that deal during his first term in office, reimposing sanctions and leading Iran to dramatically increase its nuclear activity. Mr. Trump attacked Iran last June and again this year to block its progress toward a potential atomic bomb, which Iran denies seeking.

Critics say that Mr. Trump could have avoided a costly war had he left the 2015 agreement, formally known as the Joint Comprehensive Plan of Action, in place. They also warn that Mr. Trump may wind up accepting terms little better than the ones Mr. Obama secured more than a decade ago.

In a social media post on Monday, Mr. Trump trashed the 2015 agreement as “one of the Worst Deals ever made having to do with the Security of our Country,” and “a guaranteed Road to a Nuclear Weapon” for Iran. He also ridiculed it for delivering $1.7 billion in cash-filled pallets to Tehran.

Why did President Obama want a deal with Iran?

Iran has insisted that its decades-old nuclear program is for peaceful purposes such as research, medicine and energy. But once established, a peaceful program can be expanded for military use.