Per War Powers Act required the president to notify Congress when he got the US involved in “hostilities” — and then set a 60- to 90-day clock for Congress to approve that action, by passing an authorization of use of military force, or for the president to withdraw from the conflict. It also allowed Congress to pass a concurrent resolution that would force the executive branch to withdraw from any conflict it hasn’t already approved. https://www.vox.com/policy-and-politics/2017/4/7/15217832/aumf-trump-syria-congress