Read more of Slate's coverage of the Libya conflict.
Congress has known about this pattern of presidential unilateralism for some time and done little in response. It has never impeached a president for using force in this way. It has continued to finance an enormous standing military force in the face of this practice. And it has done practically nothing by statute to push back on the president's power to initiate military action with that standing military force. Not even the famous War Powers Resolution of 1973 does much to address the unauthorized initiation of force by a president. It requires the president to submit a report to Congress within 48 hours whenever armed forces are introduced "into hostilities or into situations where imminent involvement in hostilities is clearly indicated by the circumstances." After the president reports the introduction of forces abroad, the resolution requires him to withdraw those forces within 60 days (or 90 days, based on military necessity) unless Congress has authorized continued operations.
The WPR does not, as some claim, authorize presidential military action within its 60-day (or 90-day) window. But it does acknowledge an inherent presidential power to use military force within that window. As former Office of Legal Counsel chief Walter Dellinger explained in his official legal justification for the planned 1994 intervention in Haiti, the WPR, by requiring quick notice to Congress and termination after 60 or 90 days, "recognizes and presupposes the existence of unilateral Presidential authority to deploy armed forces" during that period. The WPR's "structure makes sense," he explained, "only if the President may introduce troops into hostilities or potential hostilities without prior authorization by the Congress." Dellinger acknowledged that "the WPR announces that, in the absence of specific authorization from Congress, the President may introduce armed forces into hostilities only in 'a national emergency created by attack upon the United States, its territories or possessions, or its armed forces,' " but noted that "even the defenders of the WPR concede that this declaration—found in the 'Purpose and Policy' section of the WPR—either is incomplete or is not meant to be binding," especially since the WPR states that it is not "intended to alter the constitutional authority of the … President."
In light of the long pattern of presidential unilateralism, Congress' continued funding of a standing army in the face of this practice, and only very qualified restrictions in the WPR, it is hard to conclude that President Obama has acted unconstitutionally in his actions thus far in Libya. The best argument for the contrary conclusion is that no American lives or property, and no national security threat, is at stake; the Libya action seems purely humanitarian. Even if that were all it was, there are recent precedents for action, most notably Kosovo, but also Somalia, Haiti, and to some extent Bosnia. President Obama, moreover, indicated that the mission was more than humanitarian when he said that without it the "entire region could be destabilized, endangering many of our allies and partners" and the "words of the international community would be rendered hollow." This last factor might be relevant because, as many executive branch legal opinions going back to the Korean War have maintained, the United States has a national security and foreign relations interest in effectuating the U.N. system that is implicated here, and the president may take that into account in deciding to use force.
The constitutional question will become much harder if the military action in Libya approaches the 60- or 90-day limit of the WPR without congressional authorization. (Congress should be careful about how it appropriates for Libya: In 2000, the Clinton Office of Legal Counsel opined that, despite the WPR's specific proviso that authorization to continue hostilities after 90 days cannot be inferred from a congressional appropriation, Congress had in fact authorized the Kosovo intervention in an appropriation, and that this last-in-time indication of congressional intent trumped the earlier WPR.) Until he bumps up against the 60- or 90-day limit, the president can feel safe that he is acting constitutionally without getting Congress' formal approval.
Which is not to say that he is acting wisely. There are powerful political reasons for presidents to seek congressional support, and many political risks from not doing so, especially if the military exercise drags on or goes badly. The Constitution establishes this system of political incentives, which causes most presidents most of the time (for example, George W. Bush, twice) to avoid large-scale or extended military actions abroad without first securing congressional approval. It does not appear that President Obama gave the issue of domestic political support much thought when he turned on a dime last week. This is an astonishing oversight, if it was that, from a man who campaigned on the need for retrenchment and prudence in the use of U.S. military force.