The Army is the oldest branch of the U.S. military, created in 1775 to fight in the American Revolutionary War. Over the course of its existence, the U.S. Army has grown in size, structure and availability. In the modern era, the Army is responsible for most land-based combat operations undertaken by the United States, as well as peacekeeping exercises and multinational cooperative military activities.
In the era between its conception and 1917, the U.S. Army mainly operated as a militia. After the Revolution and the War of 1812, it was primarily called upon only when needed in combat against Native Americans and border defense as U.S. territories expanded towards the west. At the onset of the United States' involvement in World War I in 1917, the Army's size and responsibilities were expanded to include a constant, standing force, as well the potential for additional expansion at the discretion of the president.
As the Army grew, it was separated into specialized divisions, including active and reserve components, a National Guard and an air unit that eventually separated to become the U.S. Air Force. During the second half of the 21st century, the United States became heavily involved in activities with NATO and the United Nations. Cooperative military operations with these multinational organizations became a major responsibility of the U.S. Army.
As U.S. operations increased overseas, the Army also has maintained ongoing peacekeeping exercises, including the occupation of foreign countries when deemed necessary and the maintenance of foreign bases. As of 2015, the U.S. Army is the largest branch of the military. Its additional tasks include border security, assistance to allies of the United States and fighting the flow of the international drug trade.