In tonight’s State of the Union, President Obama called for a congressional resolution authorizing his use of military force against the ISIS terrorist organization, which has seized control of large parts of Iraq and Syria. This is a step in the right direction, because US military intervention against ISIS amounts to a war that requires congressional authorization under the Constitution. A congressional AUMF might also prevent further violation of the War Powers Act of 1973, which requires congressional authorization for military actions abroad that last more than 60 days (as this one already has). Whether he intends it as such or not, the president’s request is at least an implicit acknowledgement that the president requires congressional approval to initiate wars.

Like Obama, I believe there is good justification for military action to prevent ISIS from consolidating its control of large parts of the Middle East, and from committing further barbaric atrocities. I therefore hope Congress does pass the AUMF.

But after-the-fact authorization is not enough to cure the president’s repeated violation of the constitutional requirement that the initiation of war requires advance congressional approval. As then-senator Obama put it in 2007, “The President does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation.” Sadly, the war against ISIS is not the first time that Obama undermined his own constitutional principles on this score. He previously did it by launching a military intervention in Libya in 2011. As prominent liberal constitutional law scholar Bruce Ackerman points out, Obama’s violations of the Constitution in this respect go far beyond anything done by George W. Bush. Virginia Democratic Senator Tim Kaine has also stated that lack of congressional authorization makes the war against ISIS “illegal.”

The administration has presented a variety of shifting legal justifications for the war against ISIS. But none of them make much sense. The same is true of alternative rationales offered by such unlikely Obama allies as former Bush administration official John Yoo. Perhaps even worse, nearly all of these arguments give the president a near-blank check to initiate a variety of wars without advance congressional authorization.

Enforcing the constitutional norm against unilateral presidential initiation of wars is not an easy task. In this case, it is not helped by the fact that President Obama has twice gotten away with violating it without paying any significant political price. If Congress does give the president his AUMF in this case, it should at least couple it with a resolution indicating that the president acted illegally when he failed to get authorization in advance, and emphasizing that future interventions must have such authority.

Such a statement won’t by itself fix the problem. Far from it. But it will at least put Congress on record supporting constitutional constraints on presidential power to initiate war, and weaken the potential precedential effect of the president’s actions. Both courts and executive branch officials often cite congressional acquiescence to past presidential actions as precedents justifying future ones. Congress should at least block the establishment of such a precedent here.