Washington - When Abu Bakr al-Baghdadi declared his terrorist Islamic State, he ignored a warning from Osama bin Laden that jihadists should be cautious about establishing a caliphate too quickly. In torching a firestorm in Iraq and Syria, Baghdadi has united his enemies and given them a target to attack, just as bin Laden predicted.

Baghdadi's bloodbath has achieved the impossible: He has provided a common adversary for Saudis and Iranians, Turks and Kurds. He has united many of Iraq's Sunni, Shiite and Kurdish politicians behind an inclusive new government. He has forced a reluctant President Obama to come halfway off the bench in authorizing airstrikes for "limited military objectives" in Iraq.

The counterattack against the Islamic State that began last weekend could last "months if not years," in the vague phrase used by U.S. and Iraqi officials. American power will be essential in this rollback, but Obama was right to warn Monday, "There is no American military solution to the larger crisis in Iraq."

"This is the last chance for Iraq," Kurdish leader Massoud Barzani said Monday in a video interview with the Aspen Strategy Group, at a foreign-policy gathering I attended. "While we need your military support," Barzani said, referring to the airstrikes and advisers Obama has authorized, "we will never ask you to put boots on the ground to fight for us."

As Obama must understand, America has returned to the slipperiest slope on earth. He may seek a limited military involvement, but Baghdadi's Islamic State gets a vote. It will use suicide bombers against American targets anywhere it can find them. With its allies, it will try to attack the U.S. homeland. What began last weekend as an attempt to rescue Iraqi refugees on a mountaintop will likely have to expand.

Obama stepped gingerly into this fight, dropping humanitarian supplies, then bombing IS mortar positions and hitting convoys. In an interview to Tom Friedman of The New York Times, he urged for Iraq the spirit of "no victor, no vanquished." That's a sensible call to compromise, but it was somewhat discordant alongside the president's explanation that he had used American power in Iraq in a situation "in which genocide is threatened."

Obama's strategy may be to deter the Islamic State from attacking the West, by focusing on the defense of U.S. personnel and rescue of Iraqis - and threatening massive drone attacks and targeted killings if they cross his line. But deterring terrorists is a risky course, because it counts on their rationality.

If Obama wants to send a signal that he's serious about helping a new Iraqi government, he should consider sending retired Gen. David Petraeus and former Ambassador Ryan Crocker - the two Americans who probably know Iraq best - to Baghdad as his special envoys.

Obama has gotten the broad outline of America's re-entry into Iraq right, by insisting that America will use its power in support of an inclusive Iraqi government that unites to battle the Islamic State. In practice, that will mean arming militia groups that fight under an Iraqi banner, including the peshmerga  and a new Sunni "national guard," which two senior Iraqi Sunni leaders recommended in a separate video interview Sunday night with the Aspen Strategy Group.

"In the beginning, [Sunni] people were very sympathetic to the Islamic State as defenders of the Sunnis," explained one of the Iraqi leaders. "Now they see that they are al-Qaeda, exploding mosques and killing people." Said a second Sunni official: "They are the enemy of all."

This turn away from the Islamic State is just what bin Laden warned might happen if his followers were seen to be killing fellow Muslims in their grab for power. In a document found in Abbottabad, Pakistan, after he was killed in 2011, bin Laden cautioned that such red-hot tactics "would lead us to winning several battles while losing the war at the end."

Bin Laden thought Yemen was the most likely place where his supporters might declare a caliphate, but he worried they would do it too soon. An undated letter, perhaps written by bin Landen, cautioned: "We want ... to establish an Islamic State, but first we want to make sure we have the capability to gain control of it. Even though we were able to militarily and economically exhaust and weaken our greatest enemy before and after [Sept. 11, 2001] the enemy continues to possess the ability to topple any state we establish."

Baghdadi couldn't wait. His fighters ruthlessly seized the Sunni heartland of Iraq. Now we'll see if bin Laden's estimation of American power remains correct.

(c) 2014, Washington Post Writers Group