Yesterday, President Obama was not inclined to argue with Congress over Iran. The White House announced that, while the president thought it was unnecessary, he was going to let the Iran Sanctions Act become law without taking any action. The bill, which passed the Senate by a 99-0 vote, was largely symbolic, since everyone knew the possibility of Obama re-imposing the restrictions on economic activity with Iran it promises is an empty threat.

But the significance of the act may become clearer after January 20. If Donald Trump is as serious about revisiting the issue as he has claimed to be, then it may be the starting point for a genuine effort to restrain Iran.

The president seems to be confident that the nuclear deal will not be torn up by Trump, which is why he didn’t bother vetoing a bill that included language expanding the scope of previous efforts. The sanctions can be put into effect not just in relation to the deal but also due to other forms of Iranian misbehavior, including its illegal testing of ballistic missiles and supporting international terrorism–issues the president and Secretary of State John Kerry deliberately left out of the nuclear pact.

That is exactly where Trump can start. By demanding Iran cease work on its missile program and end support for terrorist groups, Trump would not be so much renegotiating the terms of an agreement that he rightly derided as “the worst deal ever” as he will be starting a new negotiation on which the U.S. will have the moral high ground. If the Iranians refuse, Trump can not only put back into effect the U.S. sanctions that Obama has refused to enforce; he can impose new, even tougher measures.

The argument against revisiting the nuclear deal is that, without the cooperation of America’s European allies, any such efforts are doomed. Since neither U.S. allies nor rivals like Russia and China has any interest in a confrontation with Iran, the assumption is Trump has no chance to change things no matter how much he has railed against the deal.

But supporters of the deal—which will allow Iran to go nuclear within a decade even if it doesn’t violate the terms of the pact—are underestimating the power of a U.S. Treasury Department that will enforce the law. Even acting alone, expanded sanctions—which would face minimal opposition in a Congress controlled by Republicans—that would impose penalties on any financial entity doing business with Iran could effectively re-impose international sanctions even without the consent of the Europeans.

The pieces are in place for a resetting of the foreign policy chessboard. The bill that Obama thought too meaningless to bother vetoing could actually be the start of a reversal of fortune for the Iranians.