The job will require a difficult balancing act that will be familiar ground to the retired four-star Marine officer, who has plenty of experience managing unwieldy coalitions and navigating the volatile politics of the Middle East.
    
Allen, 60, has been an unabashed hawk when it comes to Islamic State, urging a no holds-barred assault on the militants, who have employed brutal tactics in their advance across Syria and Iraq.
    
"The Islamic State is an entity beyond the pale of humanity and it must be eradicated. If we delay now, we will pay later," Allen wrote in Defense One last month.
    
As head of the NATO-led mission in Afghanistan from July 2011 to December 2013, Allen had to deal with the notoriously mercurial president Hamid Karzai - as well as commanders from dozens of countries while overseeing the start of a troop drawdown.
    
And before that, as the number two at US Central Command, which oversees American forces in the Middle East, Allen devoted much of his time to tracking America's arch-foe Iran.
    
"In this role General Allen will help continue to build, coordinate and sustain a global coalition across the multiple lines of efforts to degrade and ultimately destroy ISIL," said State Department deputy spokeswoman Marie Harf, referring to the jihadists that have seized territory in Iraq and Syria.
    
President Barack Obama, in announcing a strategy to destroy the group, said the creation of an international coalition that included Arab and Muslim states was vital to the anti-IS effort.
    
Allen's deputy will be Brett McGurk, the deputy assistant secretary of state for Iraq and Iran.
    
The former commander will not oversee military operations, which is the job of the current chief of Central Command, General Lloyd Austin. But Allen likely will be asking members of an international coalition to contribute aircraft, ammunition, access to bases or other aid to the fight.
    
Allen is no stranger to the sectarian politics of Iraq, where he made his name from 2006-2008 in western Anbar province, forging alliances with Sunni tribes who turned on Al-Qaeda militants.
    
The approach to the Sunni tribal leaders was controversial at the time, and some fellow officers opposed the effort but it proved successful, producing the so-called "Anbar Awakening."

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