The United States Department of Defense is a government agency responsible for protecting the country from military threats at home and abroad. Its synonyms include Defense Department, DoD, War Department, and National Defense Agency. Other terms for the department include Pentagon, which serves as the headquarters for the agency, and the Military, which refers to the U.S. Armed Forces as a whole. The Department of Defense is one of the largest employers in the world, with over 1.3 million active-duty service members, 800,000 reserve personnel, and 732,000 civilian employees. Its mission involves protecting the country against cyber, space, and air threats in addition to traditional military threats.