Each of the different military services is assigned a role and domain. The Army conducts land operations. The Navy and Marine Corps conduct maritime operations, the Marine Corps specializing in amphibious and maritime littoral operations primarily for supporting the Navy. The Air Force conducts air operations. The Space Force conducts space operations. The Coast Guard is unique in that it specializes in maritime operations and is also a law enforcement agency.^[17][18]


From their inception during the American Revolutionary War, the U.S. Armed Forces have played a decisive role in the country's history. They helped forge a sense of national unity and identity through victories in the early-19th-century First and Second Barbary Wars. They played a critical role in the territorial evolution of the U.S., including the American Civil War. The National Security Act of 1947 created the modern U.S. military framework, establishing the National Military Establishment (later the Department of Defense or DoD) headed by the secretary of defense and creating both the U.S. Air Force and National Security Council; in 1949, an amendment to the act merged the cabinet-level departments of the Army, Navy, and Air Force into the DoD.^[19]


The president of the U.S. is the commander-in-chief of the armed forces and forms military policy with the DoD and Department of Homeland Security (DHS), both federal executive departments, acting as the principal organs by which military policy is carried out.


The U.S. Armed Forces are one of the world's largest military forces in terms of personnel. They draw their personnel from a large pool of professional volunteers. The U.S. has used military conscription, but not since 1973. The Selective Service System retains the power to conscript males, requiring the registration of all male citizens and residents living in the U.S. between the ages of 18 and 25.


The U.S. Armed Forces are considered the world's most powerful military, especially since the end of the Cold War.^[20] The military expenditure of the U.S. was US$916 billion in 2023, the highest in the world, accounting for 37% of the world's defense expenditures.^[21] The U.S. Armed Forces has significant capabilities in both defense and power projection due to its large budget, resulting in advanced and powerful technologies which enable widespread deployment of the force around the world, including around 800 military bases outside the U.S.^[22] The U.S. Air Force is the world's largest air force, followed by the U.S. Army Aviation Branch. The U.S. Naval Air Forces is the fourth-largest air arm in the world and is the largest naval aviation service, while U.S. Marine Corps Aviation is the world's seventh-largest air arm. The U.S. Navy is the world's largest navy by tonnage.^[23] The U.S. Coast Guard is the world's 12th-largest maritime force.^[24] The U.S. Space Force is the world's only active independent space force.^[25]

Under Department of Defense regulation, the various components of the U.S. Armed Forces have a set order of seniority.^[283] Examples of the use of this system include the display of service flags, and placement of soldiers, marines, sailors, airmen, guardians, and coast guardsmen in formation.


While the U.S. Navy is older than the Marine Corps,^[284] the Marine Corps takes precedence due to previous inconsistencies in the Navy's birth date. The Marine Corps has recognized its observed birth date on a more consistent basis. The Second Continental Congress is considered to have established the Navy on 13 October 1775 by authorizing the purchase of ships, but did not actually pass the "Rules for the Regulation of the Navy of the United Colonies" until 27 November 1775.^[285] The Marine Corps was established by an act of the Second Continental Congress on 10 November 1775. The Navy did not officially recognize 13 October 1775 as its birth date until 1972, when then–chief of naval operations Admiral Elmo Zumwalt authorized it to be observed as such.^[284]


The Coast Guard is normally situated after the Space Force, but if it is moved to the Department of the Navy, then its place in the order of precedence changes to being situated after the Navy and before the Air Force.^[284]