The reason, as I have written before, is that no president—not Barack Obama and not Donald Trump—has the authority under the Constitution to “declare war.” Of all the toxic constitutional developments of the Obama years, by far the most disheartening is this: Obama’s unlawful intervention in Libya garnered strong criticism; but the harshest criticism came when Obama chose to obey the Constitution by asking for congressional authorization to strike Syria. For breaking the mold of presidential unilateralism, he garnered—and continues to garner—the undisguised scorn not only of his political enemies but even of many of his friends. That hostile verdict on his presidential leadership is the clearest sign that we have entered what future historians may describe as a post-constitutional era.

Why, of all the many military misadventures into which Uncle Sam has blundered since 2001, is Syria different? The reason is that, under the Constitution and the War Powers Act, the president has no authority to send military forces into hostilities except after congressional authorization or in response to a direct attack on the U.S. or its forces. The president has no inherent power over war; it is given to Congress. In 2001, George W. Bush grumbled about his supposed executive authority, but went to Congress for approval of a “war” against “those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001”; in 2002, he did the same again, and got approval to “defend the national security of the United States against the continuing threat posed by Iraq; and … enforce all relevant United Nations Security Council resolutions regarding Iraq.” Those resolutions remain, at least technically, in force, and have been used by the Obama and Trump administrations as justification for U.S. efforts on behalf of forces—including some of the Syrian rebels—fighting against the Islamic State, supposedly a “successor” to Al Qaeda.