One of the most durable arguments for not responding as forcefully as possible to al Qaeda, the Islamic State, and jihadi groups in general is that they do not pose an “existential” threat to America. Indeed, this lies at the core of the Obama administration’s strategy for the Middle East. As the president told CNN’s Fareed Zakaria back in February: “What I do insist on is that we maintain a proper perspective and that we do not provide a victory to these terrorist networks by overinflating their importance and suggesting in some fashion that they are an existential threat to the United States or the world order.”

Such apparent sang froid is supposed to be a hallmark of “realism” in foreign policy.  Thus Harvard political scientist Stephen Walt blogged in response to last week’s Paris attacks that “we must respond with our heads and not just our hearts,” and that “an event like this cannot shake the foundations of society unless we let it.”