The United States is withdrawing from the Iran nuclear deal.

In a speech delivered in the Diplomatic Room of the White House, President Donald Trump harkened back to his 2016 campaign rhetoric, describing the 2015 “plan of action” between Iran, the U.S., the European Union, and the United Nations aimed at hindering and monitoring Iran’s nuclear ambitions as “horrible,” “one-sided,” and “disastrous.”

Under the terms of that deal, the U.S. is permitted to reinstate sanctions if it alleges that there has been significant non-performance of the obligations of another participant to the deal. In that case, the old sanctions can “snap back” into place. And this is what Trump announced Tuesday will now happen.

In order to understand what this will mean, it helps to know a bit about how the history of Iranian sanctions, how the nuclear deal agreement worked, and what has happened since Trump took office.

Trump’s promise to cancel the Iran nuke deal was similar to promises he has made about NAFTA and trade with China, two other deals he has described in harsh language and promised to renegotiate or cancel. Each time, Trump — and by extension, the millions of Americans who voted for him — was saying that the costs of the current deal were too great and the potential rewards of gambling for a better deal were worth the risks that would have to be borne.