It’s a bullish time for executive power. President Donald Trump’s conception of it is so expansive that he has asserted that he can pardon himself. The Supreme Court has reinforced that conception by upholding Trump’s blatantly anti-Muslim executive order restricting immigration. And Judge Brett Kavanaugh’s probable confirmation will produce a decisively right-leaning court likely to be deferential to a Republican president like Trump. Trump’s belief that presidential authority is practically monarchical, his belligerent posturing toward countries such as Iran and North Korea, and his cavalier disregard for legal procedure have made many observers wonder if he will try to start a catastrophic war, and what safeguards exist to constrain him if he does.

In November 1973 Congress passed the War Powers Act, over President Richard Nixon’s veto. The Vietnam War, which had begun as an advisory mission twenty years earlier, had turned disastrous in the hands of the willful and self-deceiving Johnson administration and then the cynical Nixon team. The act was meant to give the legislative branch a measure of control over military engagements by constraining the president’s power to use military force without a congressional declaration of war or other statutory authorization.

Under Article I of the Constitution, only Congress can declare war. But Article II makes the president the commander-in-chief, and in that capacity presidents have initiated and sustained armed conflict without explicit congressional permission. Neither Korea (a “police action,” as Truman put it) nor Vietnam was a declared war, yet over 100,000 Americans and millions of Asians died during them.

The War Powers Act was designed to preclude that kind of escalation. Under it, the president can still deploy US forces before obtaining congressional approval. But the White House has to notify Congress within forty-eight hours any time American troops and other military assets are readied for combat or engaged in hostilities, including in self-defense. After that, the US cannot stay in the conflict for more than sixty days (with an additional thirty days for withdrawal) unless Congress declares war or provides statutory authorization. In effect, then, the president can authorize tactical military action without prior congressional approval in response to an emergent attack or threat, but must obtain such approval for any ensuing military build-up or strategic escalation.

In addition, “where imminent involvement in hostilities is clearly indicated by the circumstances,” the act requires the president to consult Congress before undertaking military action. In this respect, the War Powers Act arguably bars preventive military action—i.e., action to preclude an attack that an adversary is capable of but that is not imminent—without prior authorization from Congress.

Over the past forty-five years, the effective scope of the act has remained more or less the same. Hawks don’t want to press federal courts to clarify how widely it applies for fear of a ruling that would restrict executive power. Doves are afraid the courts will expand it. The courts themselves are…