Trump can’t make war whenever he likes

But the president’s speech gave no indication that he will in fact respect his decision last year to remain faithful to the 1973 law. Worse yet, in his follow-up, the defense secretary, James Mattis, advanced a much more aggressive position, asserting that “the president has the authority under Article II of the Constitution to use military force overseas to defend important U.S. national interests.”

This is precisely the view notoriously advanced by John Yoo during the early years of President George W. Bush’s administration. If Mr. Trump fails to notify Congress, as required by the 1973 statute, he will be repudiating his own precedent and embracing an extreme position that even Mr. Bush rejected during the second term of his administration. To be sure, Mr. Yoo made his sweeping claims to defend the commander in chief’s authority to torture terrorists, while Mr. Mattis is making Yoo-like arguments to support the noble cause of suppressing chemical warfare. But once Mr. Mattis’ sweeping assertion of presidential power is accepted, it will be used as a precedent by Mr. Trump to ignore the War Powers Resolution whenever he, and he alone, is persuaded that “important national interests” justify future military adventures.

Defenders of presidential war-making back up their position by claiming that Congress has abdicated its constitutional responsibility to serve as the ultimate arbiter over war and peace. But this is false. In passing its omnibus appropriations bill only last month, Congress was careful to provide that “none of the funds made available by this Act may be used with respect to Syria in contravention of the War Powers Resolution, including for the introduction of United States armed or military forces into hostilities in Syria.”

Congress even more recently responded to Mr. Trump’s military escalation in the Middle East by reinvigorating its authority under the War Powers Resolution. Its reassertion of war-making power was provoked by Mr. Trump’s unilateral military support for Saudi Arabia’s war against the Houthi rebels in Yemen. Earlier this month, a bipartisan senatorial coalition invoked a special provision of the War Powers Resolution to force a floor vote condemning Mr. Trump’s action. While this motion was returned to the Foreign Relations Committee by a margin of 55 to 44, it was only after the chairman of the Foreign Relations Committee, Bob Corker, promised to consider legislation this week that could limit the president’s war-making authority against Islamic militants.

Mr. Corker should rapidly follow through on his commitment if Mr. Trump fails to provide Congress the required notice under the War Powers Resolution. If Mr. Corker hesitates, members of the Senate and the House should fill the gap, and follow the example recently set in the Yemen case. This not only allows them to invoke a special provision of the resolution to force a timely vote on Mr. Trump’s Syrian campaign; it also prevents any efforts to filibuster or delay their initiative, since the War Powers Resolution requires a final vote “within three calendar days” after members have forced their motion onto the floor of each house.

The real challenge will come if Mr. Trump refuses to follow sober legal advice and explicitly endorses Mr. Mattis’s celebration of the powers of the commander in chief. At that point, the House speaker, Paul Ryan, and the Senate majority leader, Mitch McConnell, will face their moment of truth. They should be prepared to make it clear to the president — first privately, then publicly — that they will no longer block demands for his impeachment if he defies the Constitution, and the War Powers Resolution, and insists on his authority to make war whenever he likes.

If they refuse, they will go down in history as Mr. Trump’s obedient servants in the escalating campaign to transform the presidency into an instrument for arbitrary assertions of military force.