Read more of Slate’s coverage of the Libya conflict.

Several days into a campaign of air and sea strikes against Libya, I agree with many of the arguments from critics of the intervention: President Obama acted imprudently in committing American forces to a conflict with an ill-defined national security justification. It is unclear how, on balance, a third war in a Muslim country helps our foreign policy goals. It is uncertain that the intervention will produce a regime more to our liking than Qaddafi’s. It is hard to justify military action in Libya while the United States does not use military force in the face of brutal crackdowns by allies elsewhere in the Middle East. And it was especially unwise not to explain this action to the American people in advance or to better consult with and seek formal authorization, or at least political support, from Congress.

But that said, I depart from the critics of the Libya action, and from Sens. Obama and Hillary Clinton themselves circa 2007, and from the academic writings of Legal Adviser to the State Department Harold Koh on this one point: I do not believe that the military action in Libya is unconstitutional.

Legal scholars disagree about the original meaning of the Constitution’s conferral on Congress of the power “to declare war.” Many contend it required Congress to formally approve all uses of U.S. military force abroad, save, as James Madison said at the Convention, in situations needed to “repel sudden attack.” Others maintain the “declare war” clause provides more leeway, allowing the president to use force abroad as long as the force does not rise to the level of “war,” whatever that means. Yet others argue that the framers meant simply to give Congress the authority to signal under international law a state of war; the real work in controlling presidential initiation of force, under this view, was Congress’ control over appropriations and the size of the standing army. There are many more theories about the original understanding. Even if we could definitively resolve this debate, which we can’t, it is unclear why original intent—which in practice rarely determines contemporary constitutional meaning—should control outcomes in the context of presidential war powers, a context that as much as any is marked by radically changed circumstances.

Compounding the problem of indeterminate constitutional language is the fact that the courts have never resolved the question about the scope of the president’s power to use military force abroad without congressional authorization. Almost all litigation seeking to resolve whether a war was properly launched has been dismissed as a “political question” or because the plaintiff lacked standing. As a result, the constitutional issue has been worked out almost exclusively by practice between the political branches and not by the courts.

That practice confirms that the president, under his commander-in-chief and other executive powers, has very broad discretion to use U.S. military force in the absence of congressional authorization. Presidents have done this, in military actions large and small, over 100 times, since the beginning of the republic. The largest and most consequential unauthorized military action is the Korean War launched by President Truman in 1950. Another big conflict without congressional authorization—and, indeed, in the face of an overt congressional vote that declined to provide such authorization—was President Clinton’s Kosovo intervention in 1999. Some less significant unilateral uses of military force in the past 30 years include Haiti (2004), Bosnia (1995), Haiti (1994), Somalia (1992), Panama (1989), Libya (1986), Lebanon (1982), and Iran (1980). The executive branch has issued public legal opinions explaining the constitutional basis for most of these actions. (Some are listed here.)

Critics will claim that a pattern of consistently violating the Constitution cannot remedy the illegality of these actions. But that is not the right way to view this pattern. An important principle of constitutional law—especially when the allocation of power between the branches is at issue—is that constitutional meaning gets liquidated by constitutional practice. As Chief Justice William H. Rehnquist explained in his opinion in Dames & Moore v. Regan: “[A] systematic, unbroken, executive practice, long pursued to the knowledge of the Congress and never before questioned … may be treated as a gloss on ‘Executive Power’ vested in the President by § 1 of Art. II. Past practice does not, by itself, create power, but long-continued practice, known to and acquiesced in by Congress, would raise a presumption that the [action] had been [taken] in pursuance of its consent.”

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.