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 ISIS. 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 Authorisation for Use of Military Force, which gives the president authority to act "against those nations, organisations, or persons he determines planned, authorised, committed, or aided the terrorist attacks that occurred on September 11, 2001, or harboured such organisations or persons". The administration has also cited the 2003 AUMF that authorised 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 ISIS because, while ISIS 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 ISIS 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 organisation of ISIS, which is mostly based on confidential information that cannot be evaluated by the public. But this legalistic wrangling is beside the point. In 2011, 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. In 2013, 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: 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 US 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.

Loading Slate Eric Posner, a professor at the University of Chicago Law School, is author of The Twilight of International Human Rights Law.