President Barack Obama announced Secretary of Defense Chuck Hagel's resignation at the White House Monday morning. Obama called Hagel "exemplary" and "a great friend of mine" who helped steer the military amid a time of great transition at the Pentagon. Hagel is stepping down amid criticism of the president’s national security team on a series of global issues, including the threat posed by the militant group known as ISIS. Hagel, the only Republican on the president's national security team, has served in the job since February 2013. A successor will be named "in short order," an official said to NBC News, but Hagel will stay in the job until his replacement is confirmed.