Late in the summer of 2013, President Barack Obama pulled back from his announced plans to use unilateral military force against Syria and stated that he would instead seek Congress’s approval. “I believe our democracy is stronger when the president acts with the support of Congress,” and “America acts more effectively abroad when we stand together,” he said. “This is especially true after a decade that put more and more war-making power in the hands of the president … while sidelining the people’s representatives from the critical decisions about when we use force.”

Congress never authorized Obama to use force in Syria, and Russian President Vladimir Putin gave him an out by brokering a deal to eliminate Syria’s chemical weapons. But Obama’s statement on the need for congressional consent, and the noted contrast with his predecessor, are nonetheless clarifying in their irony.

Obama’s predecessor asserted very broad presidential prerogatives in other military contexts, and his Justice Department wrote expansive legal justifications for unilateral war. But in the context of initiating war, Bush acted in a manner respectful of separation of powers. He did not “sideline the people’s representatives from the critical decisions about when we use force.” To the contrary, he sought and received legislative authorization before using force. Both of Bush’s major wars—against the 9/11 perpetrators (and their protectors), and against Saddam Hussein's regime in Iraq—were clearly and formally approved in advance by Congress in, respectively, its September 2001 and October 2002 authorizations for the use of military force (AUMFs).

And it is Obama, not Bush, who has proven the master of unilateral war. Because of his lofty rhetoric about principle, because he sometimes appears to be a reluctant commander-in-chief, and perhaps because his claims of legal authority have been advanced and defended by lawyers who did not bring to office a reputation for hardline executive supremacy, the war powers precedents Obama has established have not been appreciated. Yet for those same reasons they will be especially credible, and thus especially tempting, to future administrations. These precedents will constitute a remarkable legacy of expanded presidential power to use military force.

The Constitution does not precisely allocate war powers, and courts consider most war powers questions unsuitable for judicial review. The practices of the political branches—the way presidents and congresses behave during war, and the manner in which presidents justify how they exercise war powers—have therefore been vital in defining the scope of presidential war powers. These precedents are invoked in both legal and political debates about uses of force.