Therein lies the rub. More than 90 days have elapsed since the Libya mission began, and Congress has not passed a law authorizing the mission or extending the 90-day clock. But the clock starts to tick only when troops have been “introduced into hostilities,” and this statutory phrase is hardly self-defining. True, American planes and drones are dropping bombs. But they are not taking fire. There are no American body bags. American forces are arguably not so much “into” hostilities as “above” them.

The War Powers Resolution itself, in the very subsection setting out the 60-90 day clock, emphasizes “the safety of United States Armed Forces.” Shortly after the resolution was enacted, a key executive-branch document (PDF) defined the “into hostilities” phrase as involving situations “in which there is a serious risk from hostile fire to the safety of United States forces.”

Also, the WPR’s clock makes more sense in a classic Vietnam-style ground war—the context in which the act arose. The act is silent about whether a president may ever reinitiate hostilities on, say, Day 93—perhaps in response to a fresh provocation—after withdrawing on Day 89. In a typical ground-troops operation, this sort of complete cessation and quick renewal of hostilities is hard to imagine. By contrast, in the kind of nimble multilateral air operation over Libya, the basic premises of the law are less applicable. U.S. planes can leave enemy airspace in a matter of minutes; other coalition partners can carry on strikes for several days, and then U.S. planes could re-enter—all in strict compliance with the letter of the law.