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The United States Armed Forces are the military forces of the United States. The armed forces consist of six service branches: the Army, Marine Corps, Navy, Air Force, Space Force, and Coast Guard. All six armed services are among the eight uniformed services of the United States, along with the U.S. Public Health Service Commissioned Corps and the NOAA Commissioned Officer Corps. 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. 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 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. 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. 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. 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. 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. The U.S. Coast Guard is the world's 12th-largest maritime force.