We’re not the first ones to point out such concerns. Senators Tim Kaine, a Virginia Democrat, and Jeff Flake, an Arizona Republican, have worked for years to draw attention to the absence of legal authorization for military operations in the Middle East. And Bob Corker of Tennessee, chairman of the Senate Foreign Relations Committee, made clear at a recent committee hearing that if the United States were to remain in Syria to fight Iranian proxies, “the authorizations are not there for that kind of activity.”

If the constitutional violations weren’t troubling enough, the announced plan would also put the United States in direct violation of international law. The American operations against the Islamic State and Qaeda elements in Syria were made in the defense of the United States and its allies, particularly Iraq. This was the basis for the American explanation to the United Nations of its military operations in Syria, provided by Ambassador Samantha Power on Sept. 23, 2014. That explanation was reiterated in a letter to Congress from Secretary Tillerson last fall.

Some critics have said this use of the self-defense exception is overbroad. But even if that argument was plausible before, the indefinite commitment of United States troops to hold territory in Syria cannot be defended on similar grounds.

Stabilizing the territory taken from the Islamic State is undoubtedly essential to ensuring terrorist threats do not re-emerge, and that must be a priority for the United States. But as the threat from the Islamic State diminishes and the Syrian conflict returns to a battle over whether Mr. Assad will regain full control over land held by rebels who are divided among themselves, it is no longer possible to argue that the mission is one of self-defense. If the president were to order American troops to hold Syrian territory in those circumstances, he would be ordering them to act in clear violation of the United Nations Charter.

In so clearly breaking international law, we would not just put our troops in harm’s way; we would also be licensing malevolent leaders the world over to follow in our footsteps. And we would be doing so when the world’s approval of American leadership has dropped to a new low — down to 30 percent from 48 percent just a year earlier. This course of action would significantly undermine America’s hard-earned global leadership as a champion of law-bound international action, perhaps irreparably.

For several decades now, Congress has gradually ceded its war authority to the executive branch. If it does not act now, it may lose what authority remains. Congress has to attend to its constitutional duties: Our troops and their families deserve a public debate over the precise scope of their mission if we’re asking them to put their lives on the line.

Congress must tell the president he cannot engage our troops in an illegal war in Syria. To allow this blatantly illegal action would spell the end of congressional authority over war. And it would offer incontrovertible evidence that the United States under President Trump is no longer a champion of the world order, but is ready and willing to tear it down.