Unified combatant command

A unified combatant command, also referred to as a combatant command (CCMD), is a joint military command of the United States Department of Defense that is composed of units from two or more service branches of the United States Armed Forces, and conducts broad and continuing missions. There are currently 11 unified combatant commands, and each is established as the highest echelon of military commands, in order to provide effective command and control of all U.S. military forces, regardless of branch of service, during peace or during war time. Unified combatant commands are organized either on a geographical basis (known as an "area of responsibility", AOR) or on a functional basis, e.g., special operations, force projection, transport, and cybersecurity. Currently, seven combatant commands are designated as geographical, and four are designated as functional. Unified combatant commands are "joint" commands and have specific badges denoting their affiliation.

The Unified Command Plan (UCP) establishes the missions, command responsibilities, and geographic areas of responsibility of the combatant commands. Each time the Unified Command Plan is updated, the organization of the combatant commands is reviewed for military efficiency and efficacy, as well as alignment with national policy. The services often disagree about potential UCP changes.

Each unified combatant command is led by a combatant commander (CCDR), who is a four-star general or admiral. The combatant commanders are entrusted with a specific type of nontransferable operational command authority over assigned forces, regardless of branch of service. The chain of command for operational purposes (per the Goldwater–Nichols Act) goes from the president of the United States through the secretary of defense to the combatant commanders.

The Department of Defense defines at least four types of command authority:

^‡ Currently, four geographic combatant commands have their headquarters located outside their geographic area of responsibility.

The current system of unified commands in the U.S. military emerged during World War II with the establishment of geographic theaters of operation composed of forces from multiple service branches that reported to a single commander who was supported by a joint staff. A unified command structure also existed to coordinate British and U.S. military forces operating under the Combined Chiefs of Staff, which was composed of the British Chiefs of Staff Committee and the U.S. Joint Chiefs of Staff.

In mainland Europe, Allied forces fell under the command of the Supreme Headquarters Allied Expeditionary Force (SHAEF). There was a separate command for the Mediterranean area. After SHAEF was dissolved at the end of the war, American forces were unified under a single command, the US Forces, European Theater (USFET), commanded by General of the Army Dwight D. Eisenhower. In the Pacific, the split in the location of base areas (the Philippines and then Australia, versus Hawaii and the U.S. West Coast); General Douglas MacArthur's towering ego; and Army-Navy rivalries meant that two separate commands were created.

The Joint Chiefs of Staff continued to advocate for permanent unified commands, and President Harry S. Truman approved the first plan on 14 December 1946. Known as the "Outline Command Plan", it would become the first in a series of Unified Command Plans.