In Congress’ recent debates over how and whether to authorize military force in Iraq and Syria, a central conceptual question has bubbled to the surface: Who exactly are we fighting? Is the enemy the Islamic State or the larger Jihadist ideology of which the Islamic State is simply a single nightmarish manifestation? In last night’s address to the nation, President Obama came down decisively, and probably inadvertently, on the wrong side.

The bulk of President Obama’s address last night centered on outlining a “systematic campaign” to “ultimately destroy” the Islamic State (IS)—or ISIL, as he referred to the terrorist group. For many of us, this was welcome news. But it was also old news. The decision to expand American strikes and pursue IS was extensively signaled and widely expected. Far more surprising, however, was the President’s announcement that he saw no need for formal congressional permission to intensify American military involvement.

For weeks, American actions in Iraq have been justified based on their small scope. Limited American airstrikes already prevented genocide and blunted some of the Islamic State’s most dangerous advances. But these strikes’ narrow official goals allowed the president to rest securely on his inherent constitutional powers as commander in chief. Thus, for a time, Congress was permitted to weasel its way out of taking responsibility for a war they undoubtedly supported. But as the Islamic State began supplementing its butchery of local innocents with the televised beheading of American journalists, Congress felt pressured to act. But how? Divisions emerged quickly about the nature of the enemy.

On one side stand those who see a clash of civilizations. For years, Senator Lindsey Graham has insisted that America is at war not just with Al Qaeda, but with “radical Islam.” And last week, Congressman Frank Wolfe announced he would introduce a resolution granting the President authority to use force against a broad array of specific terrorist organizations (including IS, Al Qaeda, Boko Haram, and others) as well as any other groups that “share a common violent extremist ideology with such terrorist groups.” The resolution drew immediate criticism for its breadth, but less noticed was its emphasis on ideology. For Wolfe and Graham, specific groups and organizational links are secondary. The enemy is violent Islamism.

But Graham and Wolfe speak for a relatively small number of members of Congress. Against them stands a much larger caucus, one primarily concerned with developing ways to limit the President’s war-making authority. These proposals all share a desire to define the enemy narrowly, and by reference to a specific organization. The drafts circulated by Senator Bill Nelson and Congressman Darrell Issa are clear, and they authorize war against one specific organization: the Islamic State. Future groups may pose future threats. These groups may share IS’ theology, or they might not. But that is for future Congresses to determine.

Officially, President Obama has always welcomed a “limited” authorization from Congress, so in theory he sides with Nelson and Issa. But last night, President Obama blew the debate apart. In his speech, he insisted it didn’t really matter what Congress does. “I have the authority to address the threat from ISIL,” he argued, and senior administration officials quickly explained that legal basis to be Congress’ 2001 authorization of force against the perpetrators of September 11. This is shocking, particularly because only minutes later, the same senior official admitted that “we’re not talking about the organization that attacked us on 9/11 anymore.” How can a law permitting force against those “who planned, authorized, committed, or aided the terrorist attacks that occurred on September 11” be used to attack a completely separate group like IS?

The answer, according to another senior administration official, is that the Islamic State is “the true inheritor of Osama Bin Laden’s legacy.” In other words, it is the ideology that matters, not the organization. To a certain extent, we can see why Obama came down this way—and thus why Wolfe’s bill is somewhat attractive. We live in a world in which terrorist groups constantly proliferate, fracture, and realign, and it’s difficult for Congress to keep up. The Islamic State was once part of Al Qaeda; now the two are shooting at each other, but they also shoot at us. So shouldn’t a terrorist’s ideas matter more than if he fits into a flow chart?

The problem with the President’s legal theory is not just that it guts congressional war powers, but that it also seriously hampers our ability to achieve any kind of victory. Ideologies are notoriously difficult to stamp out. They evolve and spread and can go underground. States cannot. And while the Islamic State is most definitely a terrorist group, it is also a state. It has territory under its control and centralized bureaucracy—all of which makes it more easily destroyed. As they draft and interpret their resolutions, Congress, and the president, ought to remember that.