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.

Obama’s war powers practices establish new precedents in three areas.

Constitutional War Powers. President Obama’s broad theory of unilateral war powers was first on display in Libya, where he ordered U.S. forces to join other NATO countries in massive air strikes against targets in Libya that resulted in the overthrow of Muammar Gaddafi. The Justice Department’s legal opinion in support of the unilateral Libya intervention took a broad but not unprecedented view of the United States’s interest in enforcing U.N. Security Council resolutions. More significantly, the administration maintained that such large-scale, non-consensual “airstrikes and associated support missions” did not amount to “War” that required congressional consent.

A similar rationale might have been in play for the Clinton Administration’s 1999 intervention in Kosovo, but the status of that controversial episode as a precedent was unclear. The Kosovo intervention was not accompanied by a published legal opinion, and its rationale, and indeed its legality, had been uncertain. In its Libya opinion, however, the Justice Department cited the Kosovo intervention in support of its reasoning, and used it to justify what became a seven-month intervention that resulted in the overthrow of a foreign government. The Justice Department’s Libya opinion thus brought the Kosovo rationale out of the legal shadows and probably extended it. It will stand as the major precedent for unilateral presidential war from the air.

Even as he sought congressional approval to strike Syria last summer, Obama pronounced an expansive understanding of presidential war powers. Previous executive branch legal opinions justifying presidential decisions to use force had generally required a threat to U.S. persons or property or, at a minimum, an interest in upholding a specific mandate from the U.N. Security Council or a regional treaty organization like NATO. For Syria, the Obama administration instead rested the president’s authority on national interests in regional stability and in enforcing norms against using chemical weapons. The ultimate reach of this principle is uncertain, and in this case U.S. strikes were called off. But Obama’s avowed authority could be extremely broad because the aims of promoting regional stability and upholding international norms related to international security or human rights will so frequently be present whenever the United States considers intervention.

The president also invoked broad unilateral war powers in a humanitarian intervention requested by the Iraqi government—to protect Iraqi civilians on Mount Sinjar and in Amirli—in the summer of 2014. Even if morally justified, these strikes appear to take further the controversial legal theory of the planned Syria strikes, albeit in the context of an invited intervention. The Iraq strikes stand as the clearest case to date in which a president has asserted the constitutional authority to use force for a humanitarian intervention without congressional authorization or a formal U.N. or regional organization mandate.

War Powers Resolution. Another war powers legacy of the Obama administration is the hole it blew in the 60-day limit on unauthorized presidential uses of force imposed by the 1973 War Powers Resolution (WPR). The WPR requires the president to cease using force after 60 days in any situation where U.S. forces are engaged in hostilities or likely imminent hostilities, unless Congress authorizes the mission.

The Obama administration acknowledged that this congressional limitation was constitutional. And yet it rendered the 60-day limit meaningless in many important contexts when it concluded that it did not apply to the Libya action, which never received congressional approval. The administration’s unconvincing basis for this conclusion was that U.S. forces were not engaged in “hostilities” even though they were part of seven months of air strikes that helped decimate Libyan army units and bring down the government. Some earlier precedents suggested that much smaller and sporadic uses of force might not be “hostilities,” but the Obama administration extended this argument into altogether different terrain.

Broad Interpretation of 2001 AUMF. The final leg of Obama’s war powers legacy concerns the 2001 AUMF that Congress enacted in the wake of the 9/11 attacks. The Obama administration adopted the Bush administration’s practice of applying this law—which authorizes “all necessary and appropriate force” against the nations, organizations, or persons that perpetrated the 9/11 attacks, and those who aided or harbored them—to Al Qaeda associates. For years, however, it signaled that it wanted to narrow and hopefully kill the 2001 AUMF, with an eye toward declaring the end of the war with Al Qaeda. “I look forward to engaging Congress and the American people in efforts to refine, and ultimately repeal” the 2001 law’s mandate, Obama said in a speech in May 2013. “I will not sign laws designed to expand this mandate further,” he added.

And yet, the president’s administration extended the AUMF’s mandate dramatically, and gave its most expansive interpretation, when it pronounced last month that the statute applied to the Islamic State (IS). This interpretation was controversial because IS did not exist in 2001, and though one of its predecessor organizations (Al Qaeda in Iraq) was once joined to Al Qaeda, IS and Al Qaeda broke ties in early 2014. The administration appeared to reason that IS was nonetheless covered by the 2001 law because of its long ties to Al Qaeda and Osama bin Laden, the history of conflict between the United States and its predecessor organization, and its status as “the true inheritor of Usama bin Laden’s legacy.” Scholars, pundits, and members of Congress have debated the administration’s move, with most finding it a great stretch at best, and at worst a flagrant distortion.

None of Obama’s legal positions can be viewed as radical against the historical backdrop of steady unilateral presidential expansion of the war powers. Many presidents, and all modern ones, have asserted broad powers to use military force. Obama’s legacy of expanding unilateral war powers is nonetheless significant.

War from a Distance. One of Obama’s signature policies has been to eschew the heavy footprint wars of his predecessor, and to switch instead to light-footprint war characterized by small forces acting with stealth and a heavy reliance on air power, especially drones. Obama’s war powers legacy tracks this policy change.

It is always hard to identify precise limits on presidential war powers because they are so fluid and contextual. But if we take seriously the past precedents and the Obama administration’s legal theories, the position of the executive branch now appears to be that the president has the constitutional authority to use force from the air for long periods, possibly supported by special operations forces on the ground, to halt instability and uphold security or human rights treaties, sometimes in the absence even of a self-defense rationale or United Nations or regional security organization support. The executive branch also appears to believe that these things can be done to a great extent without implicating the War Powers Resolution. The president’s objectives in crafting these legal doctrines may be worthy, but he has arrogated to himself and future presidents new powers to pursue them militarily.

Ad Hoc Legal Decisionmaking. For all of Obama’s lofty rhetoric about the principles of American democracy, his practice on war powers reflects a relentlessly short-term pragmatism. The administration’s process for legally justifying unilateral uses of force has been events-driven and disorderly. 

The administration published a legal opinion on the use of force in Libya. Yet as the 60-day deadline in the War Powers Resolution loomed, it scrambled to explain how the use of force could continue. As the New York Times reported, the normally decisive Office of Legal Counsel (OLC), backed by the top Defense Department lawyer, advised the president that the WPR required a cessation of hostilities in Libya; but in a rare move, Obama overrode OLC and relied on advice of his White House counsel and State Department legal advisor to embrace the controversial “no hostilities” theory and skirt the WPR.

In addition, the administration offered imprecise and seemingly changing public legal justifications for the threatened military campaign against Syria and the campaign against the Islamic State. In neither case did it publish legal opinions. Instead, information about legal authorization dripped out in terse statements by the White House Counsel and in anonymous background discussions with journalists.

Failure of Executive Leadership. Another legacy will be Obama’s surprising failure to engage Congress and get its clear support for his military campaigns. This happened in Libya in 2011, it almost happened in Syria in 2013, and it happened again when he declined to ask for congressional authorization for the use of force against IS. Obama often talks about working with Congress to obtain its approval, but on his major war powers initiatives, he has not done so.

The president’s team has often pled the excuse that Congress will not work with him. It is certainly true that the feckless Congress has been complicit in the president’s war powers agenda by its failure to engage in meaningful oversight. But many other presidents have asked for and (after a significant political push) received congressional authorization from an apathetic or skeptical Congress, including ones dominated by the opposite party. For example, President Dwight D. Eisenhower obtained authorizations for force in Formosa (Taiwan) and the Middle East from a Congress controlled by Democrats. President George H.W. Bush did the same for Iraq in 1991.

A difficult Congress is no excuse for a lack of presidential engagement with Congress. “The Constitution is a permanent challenge to presidential leadership,” wrote Arthur Schlesinger Jr., the great presidential scholar. “It is a test of a President’s capacity to persuade Congress and the people that his policies make sense.” Schlesinger’s prescription for an uncooperative Congress was “leaders who possess not only a personal vision but the capacity to communicate that vision to their age.” These are not qualities Obama has displayed in the war powers context.

Expanding and Extending the War on Al Qaeda. As noted above, Obama and his subordinates have for years hinted that the administration wanted to narrow and repeal the AUMF and declare an end to the armed conflict against Al Qaeda. The rise of IS have belied these hopes. By extending the 2001 AUMF to IS rather than asking Congress for a new authorization against IS, a big part of his legacy will include breathing new life into that law and resting more and broader counterterrorism operations on it. The man who hoped to end the war under the 2001 AUMF, and who pledged to fight expansions of its mandate, unilaterally interpreted it to broaden its substantive reach geographically and its temporal reach far into the future.

These developments raise the stakes for what Obama does with his final two years in office, and whether his legacy will stand for something more than political expediency on war powers issues. Now that it is clear American forces will be fighting transnational terrorist organizations well into the future, will the president finally engage Congress in a serious way to establish a durable legal framework? Or will he continue to formulate ad hoc, unilateral responses as each legal issue related to war powers arises?

Obama’s legacy will look quite different if, after the midterm elections, he seeks and receives congressional authorization for the use of force against IS, especially if he also works with Congress on a framework statute that updates the 2001 AUMF to deal with the many emerging threats around the world in a principled, transparent manner with prudent limits. The president has always seemed to want to avoid doing anything that would place his fingerprints on extending the “war on terrorism.” But his fingerprints are all over such an extension already. His legacy would be significantly improved if he embeds that extension in a new legislative framework that is respectful of the separation of powers.

To engage Congress on these issues is not to commit to congressional supremacy. For those of us who see American leadership as indispensible to international security and who worry about future American declinism or isolationism, a strong presidency with broad authority to wield military force is critical. Over the past century, war powers have effectively shifted from Congress to the president in step with the vast growth in American military might and the global threats and responsibilities we face.

Formal congressional authorization for the president’s military actions nevertheless remains important not only in principle but also to discipline decision-making about military intervention and to ensure durable political backing for American strategy. Obama correctly stated last summer that “our democracy is stronger when the president and the people’s representatives stand together.” And yet, modern history shows that Congress dodges responsibility and will rarely get involved without a big push to do so from the president. In this context, only Obama can make the separation of powers work.