Report to Congress on the Department of Defense

The following is the July 31, 2026, Congressional Research Service report, Defense Primer: The Department of Defense.

From the Report

The Department of Defense (DOD) was established after World War II by the 1947 National Security Act (P.L. 80-253). At the time, some policymakers, including former President Truman, took the view that the different components of the U.S. military had been insufficiently integrated to wage World War II effectively. Congress’s stated intent was to, inter alia, “provide a Department of Defense … under the direction, authority, and control of the Secretary of Defense; … provide more effective, efficient, and economical administration; … and provide for the unified strategic direction of the combatant forces.” Since September 5, 2025, DOD has been “using a secondary Department of War designation,” under Executive Order 14347.

Over time, DOD has grown into one of the largest bureaucracies in the world, currently comprising more than 2.8 million service members and civilians stationed in over 4,800 sites across the United States and around the globe. Congress, in its constitutional role, legislates both defense authorization and appropriations bills, and conducts oversight of DOD, through budget and posture hearings. The Senate provides advice and consent through the confirmation process for multiple DOD civilian officials, including the Secretary and Deputy Secretary, and military officers, including the Chairman of the Joint Chiefs of Staff, and officer promotion selection lists.

DOD’s mission is “to provide the military forces needed to deter war and ensure our nation’s security.” It does so through five primary sets of institutions, each representing thousands of people in dozens of specific offices:

The Office of the Secretary of Defense (OSD), which is using “Office of the Secretary of War” as a “secondary” designation under Executive Order 14347 dated September 5, 2025.

The Joint Chiefs of Staff and Joint Staff.

The Military Departments.

The Unified Combatant Commands.

The Defense Agencies.

Download the document here.