In coming days, President Trump is expected to decide whether to send thousands of combat troops into Syria to attack Islamic State. A month ago, he asked the Pentagon for options on ways to “accelerate” the defeat of IS in its stronghold. If he does seek to put so many American soldiers on the ground, the commander in chief must first get the approval of Congress, where constitutional authority for war belongs.

For decades, starting during the cold war and later after the 9/11 attacks, Congress has steadily given up much of its responsibility to define the use of violence in the name of the American people. Previous presidents, both Democrats and Republicans, have broadly defined their executive power in conducting military operations in many countries. Yet a democracy must continually set clear parameters for the official use of violence, especially as threats change and weapons evolve.

Lawmakers, and not only a president, should be held accountable for the effects of warfare. Achieving and keeping peace relies on the collective wisdom of knowing when not to fight or knowing what kind of war to wage. If Mr. Trump wants to send soldiers into combat, now is the time to define that wisdom.

Few Americans would disagree with the goal of defeating Islamic State or its related militants around the world. But citizens must also have a say, through a consensus in Congress, on the means and methods as well as the scope of time and geography for such warfare. Congress should also emphasize the nonmilitary ways to help end the threat of terror, such as the work of Muslims to prevent the radicalization of their youth with the peaceful precepts of Islam.

The struggle against both IS and Al Qaeda can often seem too complex for Congress to anticipate all the limits to be set on a president. And it may be difficult to know whether a US attack on a militant group is being done strictly in “self defense.” But such issues are not an excuse for lawmakers to give up their constitutional responsibility in declaring war.

Trump is now the third president to rely on an authorization passed by Congress in 2001 to wage war on “Al Qaeda and associated forces.” That authority is very out of date. If he plans to put boots on the ground in Syria to directly attack IS, he needs to do what his defense secretary, retired Marine Corps Gen. James Mattis, suggested in 2015 in a blog.

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Mr. Mattis said that a new authorization is needed, one supported by a majority of both parties in both houses of Congress. That authority, he stated, “will send an essential message of American steadfastness to our people and to the global audience. Its passage will demonstrate our country’s fundamental unity and enable a broader commitment to deal firmly with a real and growing menace.”

The recent history of the US role in troubled conflicts, such as Iraq, Yemen, and Libya, calls for restoring full accountability in warmaking. With a new president seeking new military action, Congress can again show its proper leadership in the wise use of force.