What is the Department of Defense?
Answer
Responsible for military affairs and national defense
Explanation
The Department of Defense is the federal department responsible for national defense and the operation of the United States armed forces. Created in its current form by the National Security Act of 1947, the Department of Defense replaced the older War Department (which dated to 1789) and combined the Army, Navy, and the newly created Air Force under a single civilian Secretary. The department was reorganized again in 1949 to give the Secretary of Defense full authority over the military services.
The Department of Defense is by far the largest federal department, with roughly 2.9 million employees including active duty military personnel, National Guard and Reserve members, and civilian Defense Department employees. It controls the largest single budget of any federal department, typically more than 800 billion dollars per year in recent decades. The Pentagon, located in Arlington, Virginia, just across the Potomac River from Washington, D.C., serves as the department's headquarters. The five-sided building is one of the largest office buildings in the world.
The Secretary of Defense is sixth in the line of presidential succession, after the Vice President, Speaker, President Pro Tempore, and the Secretaries of State and Treasury. The department oversees five military services: the Army, Navy, Air Force, Marine Corps, and Space Force (created in 2019). The Coast Guard normally operates under the Department of Homeland Security but can be transferred to the Navy in wartime. Each service has its own Secretary and Chief of Staff, with the Joint Chiefs of Staff serving as a senior advisory body to the Secretary of Defense and the President.
The Department of Defense conducts military operations, manages U.S. military bases at home and around the world, develops weapons systems, runs intelligence agencies including the National Security Agency and the Defense Intelligence Agency, and supports allies through training, joint exercises, and arms transfers. The Secretary of Defense reports directly to the President as Commander in Chief and is responsible for civilian control of the military. Recent Secretaries include James Mattis, Mark Esper, Lloyd Austin, and Pete Hegseth.
Why this matters for your test
National defense is one of the federal government's core constitutional responsibilities, and the Defense Department's size, budget, and global reach make it central to American life and foreign policy.
Source: USCIS 128 Civics Questions (2025)
