The United States is now officially at war with the Islamic State in Iraq and Syria. U.S. planes began dropping bombs on targets in Syria last week. Gen. Martin E. Dempsey, the chairman of the Joint Chiefs of Staff, has told the Senate that ground troops may be needed. But President Barack Obama did not ask Congress for legal authority to go to war.

Bruce Ackerman, a professor at Yale Law School, thundered in The New York Times that Obama’s declaration of war “marks a decisive break in the American constitutional tradition,” reflecting the theories underlying “Bush-era abuses of executive authority.” Many other commentators agreed that Obama’s act was unlawful. But it’s not. The “Bush-era abuses of executive authority” reflected constitutional traditions that Obama now takes advantage of.

To judge the legality of war against ISIS, the terrorist group that calls itself the Islamic State, we need to be clear about two issues. The first is whether the president can put troops in harm’s way on his own authority. While the Constitution vests in Congress the power to “declare war,” presidents have launched military attacks on their own for many decades. Obama used military force in Libya in 2011; Bill Clinton, in Serbia in 1999; George H.W. Bush, in Panama in 1989; and Ronald Reagan, in Grenada in 1983. In all these cases, and many more (including the Korean War), Congress did not give its consent. Executive branch lawyers have ginned up an “implicit” constitutional authority to use force on one’s own — located variously in the commander-in-chief clause of Article II of the Constitution, or in the general grant of “executive power” to the president, which has been claimed to include at least limited war powers.

The second is whether unilateral force violates the War Powers Resolution, a 1973 law that Congress passed to try to rein in unilateral warfare by the president. That law provides that if the president sends troops into hostilities, he must withdraw them within 60 days unless he obtains consent from Congress. The statute obviously is in tension with the theory that the executive can use force on his own, and its constitutional status has been disputed.

The White House has not relied on Article II to justify the war on the Islamic State. This theory is too closely associated with the Bush administration, which used it to justify surveillance and torture that violated statutes. The Obama administration instead pointed to the 2001 Authorization for Use of Military Force (AUMF), which gives the president authority to act “against those nations, organizations, or persons he determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harbored such organizations or persons.” The administration has also cited the 2003 AUMF that authorized the president to go to war to “defend the national security of the United States against the continuing threat posed by Iraq,” then governed by Saddam Hussein.

The White House’s defenders argue that the 2001 AUMF gives Obama the authority he needs to fight the Islamic State because, while it broke from al-Qaeda in 2012, it is nonetheless composed of former al-Qaeda members (at least in part), who have (or so it is argued by the administration) continuously conducted and sought to conduct attacks against the United States and its citizens and interests.

Ackerman and others have pointed out that the Islamic State is not so much a successor of al-Qaeda as a competitor. It is pretty strange to interpret a 2001 law designed to give the president the power to avenge the 9/11 terrorist attack as allowing him to battle any gaggle of terrorists who are remotely associated with the original crew more than a decade later. If Congress wanted to give the president indefinite authority to fight Islamic terrorists forever, it could have said so — indeed, it rejected a Bush administration proposal that would have done just this. The interpretation also gives credence to the White House’s views about the organization of the Islamic State, which is mostly based on confidential information that cannot be evaluated by the public.

But this legalistic wrangling is beside the point. In 2011, President Obama used military force against Libya. Unable to rely on the 2001 AUMF or the 2003 AUMF against a government that had nothing to do with al-Qaeda or Iraq, Obama relied on his constitutional authority. The administration claimed that the War Powers Resolution did not apply because sending planes to drop bombs and fire missiles at enemy troops did not amount to “hostilities” governed by the war powers act. This was a ludicrous interpretation of the law. It is plain that the president will cite statutes if they exist, but if none do, that won’t stop him.

Many commentators just can’t get this into their heads. In 2013, President Obama briefly considered using military force against Syria, but when he couldn’t obtain international support, he sought an AUMF from Congress. Even though he explicitly said that he didn’t legally need that support, commentators leapt to conclusions. To quote David Rothkopf, just one of many:

“Whatever happens with regard to Syria, the larger consequence of the president’s action will resonate for years. The president has made it highly unlikely that at any time during the remainder of his term he will be able to initiate military action without seeking congressional approval.”

So much for the majesty of the law. As I argued at the time, because the president announced that he could act alone, and that he regarded resort to Congress as optional, “President Obama has reaffirmed the primacy of the executive in matters of war and peace.” He went to Congress for political cover, not for legal authority.

Ackerman is right that the Obama administration’s reliance on the 2001 AUMF is phony, but he’s wrong to say that Obama has broken with American constitutional traditions. That tradition dictates that the president must give a nod to Congress if he can, but otherwise he is legally free to go to war, subject to vague limits that have never been worked out. That’s not to say that Congress is helpless. It can refuse to fund a war if it objects to it. But the real constraint on the president’s war-making powers is not law, but politics.

It’s worth asking how this came to pass. The founders, who were suspicious of both executive and legislative power, divided the war powers by giving Congress the power to declare war and the executive the authority to lead the troops. The two branches could check each other, keeping us out of unnecessary wars. That’s no longer the case. For while Congress still holds the purse strings, it is hard to cut off funding once the president starts a war.

The answer seems to be that the United States has taken on a global role in a world in which technology makes it possible to attack and be attacked with no warning. To maintain that role, the United States must be ready to use force whenever its interests are threatened, anywhere in the world. Congress is a deliberative body and often can’t be consulted. Even if it can, it has been so taken over by parochial interests that it can’t be depended on to act in the national interest. Aware of its own limitations, Congress has created a standing army and vast intelligence system that operates under the president’s authority, which gives him the practical ability to act on his own. It has tolerated and usually endorsed military actions that the president launched on his own initiative.

It’s the U.S. role as world’s police officer that has made it necessary to allow the president to go to war without Congress. You can’t have one without the other, and so it’s this role that needs to be debated. The legal arguments are a distraction.

Eric Posner is a professor at the University of Chicago Law School.

