If you want to stop Donald Trump from making unilateral decisions regarding war and peace, then stop letting all presidents make unilateral decisions about war and peace. It’s really quite simple. Trump can abruptly pull back U.S. troops from northern Syria because Congress, having abdicated its foreign policy responsibilities long ago, has no leverage to stop him.
When Congress passed the War Powers Resolution as the Vietnam War was winding down, it gave presidents the power to send troops abroad for 60 days in response to any “national emergency created by attack upon the United States, its territories or possessions, or its armed forces.” If the president failed to gain congressional support for the deployment, he would have another 30 days to pull back troops.
Congress is the institution vested with the power to declare wars, to debate where we send troops, and decide which conflicts are funded. Presidents have been ignoring this arrangement, abuse authorizations for the use of military force (AUMFs), and imbue themselves with the power to engage in conflicts wherever they like, without any coherent endgame, and without any buy-in from Congress.
Congress, in turn, has shown no interest in genuinely challenging executive power, because its members are far more concerned with political self-preservation. Ignoring abuse shields them from tough choices and ensuing criticism—even as they use war as a partisan cudgel.