For President Obama, this is gut-check time on Iraq. He is moving the nation back onto a pitiless battlefield, with a war plan that is long on good intentions and short on clarity about the ultimate mission.

It’s a wrenching moment: A president who for several years seemed allergic to U.S. involvement in the Iraqi and Syrian wars is being drawn into this conflict by circumstances that even the skeptics agree require U.S. action. Obama kept his distance despite the deaths of 200,000 Syrians but apparently can’t do so any longer after the beheading of two Americans.

“We have to do it,” says Zbigniew Brzezinski, a former national security adviser and the dean of a group of strategists who met with Obama on Monday night. But he cautions that “because the conflict is likely to spread to other countries, and to last longer than we expect, we have to avoid the mistakes we made after September 11, 2001,” of seeming to launch a global war on terror. This time, argues Brzezinski, the United States needs to rely on its Muslim allies, such as Saudi Arabia, Turkey and Jordan, rather than making it the United States’ fight.

Obama has come gradually and reluctantly to his conclusion that U.S. military action is necessary against the self­described Islamic State that has taken root in Iraq and Syria. He’ll summarize his rationale in a speech Wednesday night. But there are some obstacles and potential dangers hidden in the fog of policy. These aren’t arguments against strong policy so much as warnings of possible unintended consequences:

● What’s the exit strategy? As Obama begins his effort to “degrade and ultimately destroy” the Islamic State, his aides told the New York Times the campaign could take three years. How will the United States and its allies know when they have “won”? Or will this be more like the Cold War, a decades-long ideological battle punctuated by periods of intense local combat? If so, are the American people ready for such a long and patient struggle?

● If Obama is serious about using U.S. military power against the Islamic State, why has he initially been so tentative? Militarily, a sudden, sharp attack makes more sense than a drizzle of airstrikes. There may be sound political reasons for the cautious U.S. approach, to force countries in the region to step up and make commitments themselves, but this goes against military logic.

● The United States may begin with the limited goal of helping allies fight the Islamic State, but what if the campaign goes badly, or it spreads more widely to Pakistan and Saudi Arabia, or the U.S. homeland is hit in retaliation? We may plan a restrained campaign, but the enemy gets a vote. Won’t the United States inevitably have to escalate if it seems to be losing?

● What about the jihadists’ haven in Syria? The United States learned in Vietnam and Afghanistan that it’s almost impossible to stop an insurgency that maintains a strong logistical base across a protected border. If the United States intends to strike targets in Syria eventually, how does it avoid becoming the air force of Syrian President Bashar al-Assad?

● Isn’t the United States implicitly allying with Iran, no matter what the two countries may say publicly? I think the answer is yes, and that this implicit cooperation is actually one of the potentially beneficial features of the campaign. But at a time when Iranian-backed extremists in Hezbollah also threaten regional stability, this is a strange brew, indeed. What needs to be de-conflicted aren’t just the two nations’ drones, but their regional policies.

● Who is going to take this fight door-to-door in the densely populated Iraqi cities of Mosul, Ramadi and Fallujah — to say nothing of Raqqah and Aleppo in Syria? The U.S. Joint Special Operations Command perfected a new kind of killing machine against al-Qaeda in Iraq. Who will be the command’s successors in the battle against the Islamic State? I suspect that U.S. Special Operations forces will have to join this fight, too, as “advisers,” or wearing different hats as CIA covert operators.

And, finally, the hardest question: Is the United States walking into a trap that has been constructed by the Islamic State — launching attacks that will rally jihadists around the world? From everything the jihadists proclaim in their propaganda, we can sense that they have been dreaming of this showdown. This is why the United States needs to make sure that, with every step it takes, it is surrounded by Muslim friends and allies.

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