The Department of Defense (DoD) is America's oldest and largest government agency. The mission of the DoD is to provide the military forces needed to deter war and to protect the security of our country. The department's headquarters is at the Pentagon. With our military tracing its roots back to pre-Revolutionary times, the Department of Defense has grown and evolved with our nation. With over 1.4 million men and women on active duty, and 718,000 civilian personnel, the DoD is the nation's largest employer. Another 1.1 million serve in the National Guard and Reserve forces. More than 2 million military retirees and their family members receive benefits.