President Trump’s decision on Tuesday to pull the U.S. out of the Iran nuclear deal and reimpose strict sanctions on the Islamic republic’s economy will begin a weekslong process to untangle the agreement.

The announcement fulfilled a key campaign promise Trump made at the very beginning of his bid for the White House: to “rip up” the 2015 agreement that was negotiated by Obama administration officials and was said to have closed Iran’s path to a nuclear weapon for at least the next decade.