Secretary of State Rex Tillerson recently announced that American troops will remain in Syria even after the fight against the Islamic State has ended. If the administration carries through on this promise, it will commit the American people to an indefinite conflict that could bring our troops into direct confrontation with the armed forces of Syria and its closest allies, Iran and Russia. In the process, it will break just about every relevant law on the books.

As repulsive as the regime of President Bashar al-Assad is, an American-backed attempt to prevent it from re-establishing control over Syrian territory, as Secretary Tillerson indicated, would embroil the United States in a new prolonged, bloody and increasingly complicated conflict. Indeed, Turkey’s cross-border air and ground assaults in recent days on Afrin, a northern Syrian enclave held by American-supported Kurdish militias, demonstrate how fragile the situation is.

To be sure, any course of action in Syria risks dire and deadly consequences. Nearly half a million people have died since the outbreak of the Syrian civil war. The horrors of the violence there defy description, and the role and intent of Russian and Iranian forces backing Mr. Assad remain of deep concern.

Yet even though the consequences of bringing American troops home after the defeat of the Islamic State in Syria are uncertain, far more dangerous are the consequences of remaining indefinitely.

Yes, the United States has a role to play in holding Mr. Assad accountable for the crimes he has committed against his own people and preventing him from committing more. But the president doesn’t have the power to unilaterally make the decision to commit American troops to stop Mr. Assad by force. He needs to make his case to Congress and the American people, as well as the international community. Inserting American troops into this situation on his own is not just bad policy, it is illegal under both the Constitution and international law.

Start with the Constitution. The founders created a system in which the power to declare war rests with Congress, not the president. The 1973 War Powers Resolution requires the president to notify Congress within 48 hours of introducing armed forces into hostilities in the absence of a declaration of war. The president must then terminate the use of armed forces within 60 days (with 30 days for withdrawal) unless Congress has declared war or otherwise authorized the use of military force.

The Trump foreign policy team has made clear that it does not think it must abide by any of these rules, and the new commitment announced by Secretary Tillerson shows that it is ready to break them. Clearly and lamentably, the president is prepared to flout constitutional limits on his authority to commit our troops on his own.

The Trump administration might argue that the troops are already in Syria, so keeping them there isn’t “introducing them.” But that would make a mockery of the statute. The Obama administration explained in 2015 that the authorizations for the use of military force that Congress agreed to in 2001 and 2002 provided the legal basis for operations against the Islamic State because they were broadly aimed at terrorism as a threat. That justification was tenuous, but surely nothing in the statutes could plausibly be said to extend to an American military presence in Syria after the Islamic State has been credibly debilitated.

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.

New York Times (United States)