The United States has replaced the current commander of American forces in Afghanistan, general David McKiernan, with lieutenant general Stanley McChrystal. McKiernan had done a good job, but it is believed that McChrystal, a career infantry and special operations officer, is more capable of innovating and developing new tactics and techniques needed to deal with the unique situation in Afghanistan. In the late 1990s, he commanded the Ranger Regiment. In 2003-6, he commanded special operations troops in Iraq, and led the hunt for terrorist leaders. He was seen as a future commander of SOCOM.
McKiernan is an armor officer, who commanded armored units until 2001, and then took over command of U.S. forces in Europe. McKiernan, when serving in Iraq, saw the importance of going after the Sunni Arab terrorists, before other senior commanders recognized this.
Relieving combat commanders in wartime is an old American tradition. In past wars, the United States entered combat with a lot of generals and colonels who had achieved their rank with no combat experience, and a background of the good behavior and cooperation needed to achieve promotions in peacetime. But in combat, you needed a different kind of personality, and the U.S. solved this problem by replacing most of those peacetime commanders with junior officers more suited for winning battles (and destined to never rise very high in peacetime.)
This changed in 1991, after new attitudes, and (much more realistic) training methods for commanders allowed those with combat leadership skills to be identified and prepared for the battlefield, in peacetime. This situation still applied in 2003, for the Iraq invasion. While the media tended to portray senior American commanders as unprepared for the fighting in Iraq and Afghanistan, foreign generals saw this was not the case. Army and marine commanders were quick to adapt to the unique conditions of irregular warfare. In the next decade you'll see revisionist historians discovering all this.
American ground forces have a deep bench. Even now, U.S. commanders realize that McKiernan, the armor officer, could do the job in Afghanistan. But McChrystal, the special operations guy, is also available, and it's believed that he can do the job faster and better. This is a rare luxury in military history, being able to replace a good general with a better one. Usually, you are lucky just to have enough competent commanders to go around.