The United States Armed Forces are the military forces of the United States. The armed forces consists 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. Each of the different military services is assigned a role and domain. The U.S. Army conducts land operations, while the U.S. Navy and U.S. Marine Corps conduct maritime operations, with the Marine Corps specializing in amphibious and maritime littoral operations in support of the Navy. The U.S. Air Force conducts air operations, while the U.S. Space Force conducts space operations. The U.S. Coast Guard is unique in that it is a military branch specializing in maritime operations and also a law enforcement agency. The president of the United States is the commander-in-chief of the armed forces and forms military policy with the Department of Defense (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 largest military forces in terms of personnel. They draw their personnel from a large pool of professional volunteers. Although military conscription has been used in the past, it has not been used in the U.S. since 1973. The Selective Service System retains the power to conscript males, and requires that all male citizens and residents residing in the U.S. between the ages of 18–25 register with the service. The U.S. Armed Forces are considered the world's most powerful military. The military budget of the United States was US$693 billion in 2019, the highest in the world. In 2018, that accounted for 36 percent 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 enables a widespread deployment of the force around the world, including around 800 military bases outside the United States. The U.S. Air Force is the world's largest air force, while the U.S. Army Aviation Branch is the second largest. 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.