McChrystal took command from Gen. David McKiernan during a low-key ceremony Monday in Kabul. McKiernan was fired by the Pentagon one year into a two-year assignment.
McChrystal, a former special forces commander, is expected to bring a more unconventional approach to a war that has turned increasingly violent almost eight years after the 2001 U.S. invasion.
The four-star general commands the largest international force ever in Afghanistan. A record 56,000 U.S. troops are in the country, alongside 32,000 forces from 41 other countries.