A few years ago, when President Barack Obama unilaterally decided to intervene in Libya's civil war, he argued that he did not need approval from Congress because bombing military targets did not constitute "hostilities" under the War Powers Resolution. That argument was so laughable that it was rejected even by the war's supporters in Congress and the press, not to mention Obama's own Office of Legal Counsel.

For a while it seemed the Obama administration was trying out a variation on that claim as an excuse for its military campaign against the Islamic State in Iraq and Syria (ISIS), which Secretary of State John Kerry repeatedly refused to call a war. But the White House quickly corrected Kerry: This is a bona fide war-just not the sort Congress has to declare.

To be more precise, Obama claims Congress already declared war on ISIS, although it surely did not realize it was doing that. Thirteen years ago, the president notes, Congress authorized George W. Bush "to use all necessary and appropriate force against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons."

It is hard to see how that Authorization for Use of Military Force (AUMF) covers ISIS, which did not exist at the time and, although it used to be affiliated with Al Qaeda, has been expelled by the latter organization. Defending Obama's interpretation of the AUMF, former Solicitor General Walter Dellinger told The New York Times you could "read the reference to 9/11 organizations to include all the evolving versions of radical jihadism." Yes, but not very plausibly.

In a speech last year, Obama argued that Congress should "refine, and ultimately repeal, the AUMF's mandate," because "unless we discipline our thinking, our definitions, our actions, we may be drawn into more wars we don't need to fight, or continue to grant Presidents unbound powers more suited for traditional armed conflicts between nation states." You can't say he didn't warn us.

Obama has a back-up justification for the war against ISIS-or at least, the part of it that is happening in Iraq. "The 2002 Iraq AUMF would serve as an alternative statutory authority basis on which the president may rely for military action in Iraq," the White House says. "Even so, our position on the 2002 AUMF hasn't changed and we'd like to see it repealed."

So here is another authority that Obama says he should not have but which he is nevertheless happy to use as a rationale to avoid a constitutionally required vote by Congress. Nor is that the only awkward part.

Obama was famously against the war in Iraq, which he called "dumb" and "rash." Three years ago the White House bragged that his promise to end that war had been "wholly fulfilled." Last June the White House said the Iraq AUMF "is no longer used for any U.S. Government activities."

By citing the Iraq AUMF as cover for the war against ISIS, Obama contradicts himself. Apparently the war he ended continues, and the obsolete authorization for it is suddenly relevant.

Obama needs a legal fig leaf to cover his blatant hypocrisy. As a presidential candidate in 2007, he declared that "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." Obama admits ISIS does not pose such a threat.

A cowardly Congress is happy to accommodate Obama's power grab. Legislators are mindful of popular support for action against ISIS but afraid to endorse a war that may end disastrously. By limiting their role to approving the arming and training of Syrian rebels, they can have it both ways, and all they have to sacrifice is the Constitution.