To judge the legality of war against ISIS, the terrorist group that calls itself the Islamic State, we need to be clear about two issues. The first is whether the president can put troops in harm’s way on his own authority. While the Constitution vests in Congress the power to “declare war,” presidents have launched military attacks on their own for many decades. Obama used military force in Libya in 2011; Bill Clinton, in Serbia in 1999; George H.W. Bush, in Panama in 1989; and Ronald Reagan, in Grenada in 1983. In all these cases, and many more (including the Korean War), Congress did not give its consent. Executive branch lawyers have ginned up an “implicit” constitutional authority to use force on one’s own—located variously in the commander-in-chief clause of Article II of the Constitution, or in the general grant of “executive power” to the president, which has been claimed to include at least limited war powers.

The second is whether unilateral force violates the War Powers Resolution, a 1973 law that Congress passed to try to rein in unilateral warfare by the president. That law provides that if the president sends troops into hostilities, he must withdraw them within 60 days unless he obtains consent from Congress. The statute obviously is in tension with the theory that the executive can use force on his own, and its constitutional status has been disputed.

The White House has not relied on Article II to justify the war on ISIS. This theory is too closely associated with the Bush administration, which used it to justify surveillance and torture that violated statutes. The Obama administration instead pointed to the 2001 Authorization for Use of Military Force, which gives the president authority to act “against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons.” The administration has also cited the 2003 AUMF that authorized the president to go to war to “defend the national security of the United States against the continuing threat posed by Iraq,” then governed by Saddam Hussein.