"The fact that Trump ordered a one-off missile strike a year ago doesn’t change that calculation. The fact that almost no one in Congress spoke up when he did doesn’t change that calculation. The fact that foreign policy commentators fawned on that decision doesn’t change that calculation. The Constitution still requires congressional authorization for an attack on another country. The requirement is not a formality. It is in the Constitution for a reason. Congress’s failure to assert its prerogatives is—even though it may have become a craven habit—a matter of life or death for a self-governing republic.
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."