If U.S. doesn't lead war on terror, who will?

Responding to the horrifying murder of photojournalist James Foley, Secretary of State John Kerry declared, "ISIL [the Islamic State] and the wickedness it represents must be destroyed." President Obama said, "people like this ultimately fail." The first is a pledge; the second an observation. Obama remains a rhetorical spectator to events in Iraq and Syria he does not want to own, and that he believes America has a limited ability to influence.

Obama called the Islamic State a "cancer." But the actual pledge found in his remarks was consistent with earlier pledges: "The United States of America will continue to do what we must to protect our people." Such a statement can be interpreted narrowly or broadly: protecting our people on the ground in Irbil against advancing Islamic State fighters, or protecting our people in New York or Washington against a terrorist threat amplified by new funding, a territorial safe haven and swelling morale. So far, Obama has given cause for the narrower interpretation.

The president wants to keep a strategic ambiguity at the center of American policy. He seems to fear that firmness will tempt our partners and allies to become free riders on American resolve. In this view, a strong U.S. commitment actually weakens the incentives for responsible behavior closer to the problem. This is the strategic insight that underlies "leading from behind."

But the current Islamic State threat - a stated desire to repeat the Foley murder on a global scale - has grown in the fertile soil of American ambiguity. The Islamic State took eastern Syria, and the U.S. did almost nothing. IS took Fallujah in January, and the U.S. did little. The group took Mosul in June, seized hard currency and American weapons, changed its name to the Islamic State and declared the caliphate, and the U.S. urged Iraqi political reform (while ramping up our intelligence capabilities). It took direct military threats against Irbil and Baghdad (and an imminent threat of genocide against the Yazidis) for America to begin limited airstrikes.

This has been a test of the doctrine of leading from behind. An American leadership "vacuum" (Hillary Clinton's word) was not filled by the resolve of friends. It was filled by Iranian adventurism, by Russian meddling, by Bashar al-Assad's mass atrocities, by Gulf state money flowing to disturbing places and by expansionist, ruthless, messianic Islamist radicalism. Recent history yields one interpretation: If America does not lead the global war on terrorism, the war will not be led.

Obama has been dragged by events toward engagement. But he still refuses to broaden his conception of America's role in the Middle East. At every stage during the last three years, he has attempted to avoid the slippery slope of intervention by defining his goals as narrowly as possible: eliminate Assad's chemical weapons, defend Americans in Irbil, prevent a genocide on Mount Sinjar. But narrowing your objectives doesn't actually narrow your problems. And denial and delay may greatly complicate such problems.

Since assuming office, Obama has taken a technical or even technological approach to the terrorist threat. If it can be narrowly defined ("core al-Qaida"), it can be surgically and antiseptically removed with drones and special operations. He is perfectly willing to take such measures: kill Osama bin Laden in his compound or strike a convoy in Yemen. But he has dismissed or downplayed the strategic and ideological aspects of the problem: Safe havens multiply threats. It is better to oppose threats aggressively and closer to their source, rather than waiting for them to arrive. Ideology and morale matter, as the Islamic State has developed momentum, attracted recruits (including from the West) and developed a reputation as the "strong horse" (bin Laden's words in 2001).

If the goal is the destruction of the Islamic State - a strategic rather than technical response to terror - allies need to be rallied to difficult, long-term tasks. Foes need to be put on notice. Americans need to be informed about the stakes and prepared for national exertions (which may eventually involve, by some estimates, 10,000 to 15,000 American troops in supportive roles).

Defense Secretary Chuck Hagel refers to the Islamic State as a threat of a "dimension that the world has never seen before." Eric Holder calls the Islamic State "more frightening than anything I think I've seen as attorney general." The central problem of American foreign policy now lies in the gap between the world's dangers and the president's diffidence.

Michael Gerson's email address is michaelgerson@washpost.com.

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