The four service branches of the United States military are the ARMY, NAVY, Air Force, and Marine Corps. The ARMY is represented by the four-letter word 'ARMY', the NAVY by 'NAVY', the Air Force by 'USAF', and the Marine Corps by 'USNAVY'. All four branches are responsible for defending the United States and its interests abroad. Each branch has its own unique mission and capabilities, and all are essential to the security of the nation.