A look at what went wrong for the four U.S. generals who have led U.S. and NATO forces in Afghanistan since 2008:
Gen. David McKiernan, June 2008 to June 2009
-Defense Secretary Robert Gates asked for McKiernans resignation a year before his term as commander was set to end. The firing was seen as a rejection by newly elected President Barack Obama of McKiernans conventional warfare approach in favor of the more targeted counterinsurgency strategy of working to undermine insurgents pull on the population.
Gen. Stanley A. McChrystal, June 2009 to June 2010
-McChrystal, who had a background in special operations, came in with a mandate to remake the war effort with the help of surge troops ordered by Obama. A year into that push, an article in Rolling Stone magazine quoted members of McChrystals team making disparaging comments about their commander in chief and other senior administration officials. Obama called McChrystal back to Washington to explain and forced him to resign.
Gen. David Petraeus, July 2010 to July 2011
-Petraeus took over the Afghan command to fill the void left by McChrystals abrupt departure and agreed to serve for one year. He completed that term and then retired from the military to become CIA director in September 2011. Petraeus resigned as CIA director on Nov. 9 after he had an extramarital affair with his biographer. The affair came out as part of an FBI investigation into suspicious emails between the biographer and another woman.
Gen. John Allen, July 2011 to present
-Allen was appointed by Obama to oversee the drawdown of U.S. and international forces ahead of the planned transfer of security responsibility to the Afghan government in 2014. Pentagon officials said early Tuesday that Allen is under investigation for thousands of alleged inappropriate communications with the second woman involved in the Petraeus case, a Florida socialite. Allens nomination to become the next commander of U.S. European Command and the commander of NATO forces in Europe has now been put on hold.