United States Department of Defense - National SecurityThe United States Department of Defense (DoD) is not only America’s oldest government agency, but also the largest. The Secretary of Defense offers direction, control, and authority over the U.S. Department of Defense. In addition, the U.S. Department of Defense is the largest employer in America with 742,000 civilian personnel and more than 1.3 million men and women serving on active duty. Additionally, 826,000 men and women serve in the U.S. National Guard and Reserve forces. There are more than 2 million retired military employees who currently receive military benefits. The U.S. Department of Defense is also responsible for protecting our country by preparing for and maintaining the national defense and security of the country.