Mark Wilson/Getty Images

On Monday, NATO's International Security Assistance Force Joint Command lowered its flag in Kabul, Afghanistan, in a ceremony formally ending the U.S.-led combat mission in Afghanistan. The U.S. and its allies invaded Afghanistan 13 years ago, to topple the Taliban and rout al Qaeda after the Sept. 11, 2001, terrorist attacks.

The end of the combat mission doesn't end U.S. military involvement in the country — starting in January, the U.S. will have some 13,000 military personnel in Afghanistan. U.S. Gen. John F. Campbell, commander of U.S. and NATO forces, says those forces will be on a training and support mission. NATO, mostly the U.S., had about 140,000 troops in Afghanistan in 2011.