The United States army was originally formed as the Continental Army on June 14th, 1775, while the Colonies were involved in the Revolutionary War against Great Britain. This unified the forces fighting the British, and was led by George Washington. Initially, ten companies of riflemen were authorized by the Continental Congress.
The first full regiment of the Regular Army was not formed until nine years later, as the 3rd Infantry Regiment. Also created in 1775 were a number of departments designed to aid the army, later renamed as the four Corps: the Adjutant General's Corps, Army Corps of Engineers, Finance Corps and Quartermaster Corps. Field Artillery was authorized in 1775, and Cavalry units in late December of 1776.
The Continental Army received aid from the Prussians, the French, the Spanish and the Dutch, and was mainly lead by officers who had fought in the British Army or colonial militias, all of which affected the Army's strategy. Their efforts culminated in the signing of the Treaty of Paris, which acknowledged the independence of the United States of America.
Because of the history of negative feelings towards large standing armies that had been fostered by the British presence in the country, the Continental Army was dissolved almost as soon as the war was over. It was mainly replaced by inconsistent, State-run militias, though a regiment and a battery of artillery remained, one to guard the Western Frontier, and one the arsenal at West Point. The continued skirmishes with the Native Americans impressed upon the people the necessity of a standing army. The first such was established in 1791, the Legion of the United States.
In 1812 America engaged in a second war against the British, this time directed against the British colony to the North, Canada. This war, the War of 1812, was less successful than the Revolutionary War. An attempted invasion of Canada failed, as did an attempt to prevent the British troops from burning the national capital building, the White House. However, an 1814 campaign, the Niagara campaign, ended in a British defeat. The war ended in an uneasy truce, and both sides returned to the status quo.
After the war, the army was mainly involved in skirmishes with Native American groups as the Americans began to pursue Westward expansion as part of the ideal of Manifest Destiny. In 1846, the annexation of Texas brought America into the Mexican-American War, which lasted for the next two years. The war ended in an American victory, and brought a great deal of new territory into the growing United States. These new acquisitions lead to even more skirmishes with the native groups located to the west of the Mississippi River.
In 1861 the Southern states seceded from the Union. The US Army was remained on the side of the Union, while the Confederate States of America formed their own army, the Confederate Army. For the first two years of the war, the army was routinely routed by the Confederate Army, but after the Union victories at Gettysburg and Vicksburg the Union army waged a successful and extremely brutal campaign against the Confederate strongholds, resulting in the Confederate surrender at the Appomattox Courthouse in April of 1865.
The Civil War was the most deadly war waged by the US Army until the Second World War, with some 360,000 casualties. After the war ended, the Army was again involved mainly in battles with the Native Americans. In the late 1890s the Army fought in the Spanish-American War, the Philippine-American War and intervened in Latin America and the Boxer Revolution in China.
1910 saw the acquisition of the US Army's first aircraft. In 1917 the US Army joined the First World War on the side of the allies. They fought on the front lines and were involved in the push into German lines. In November of 1918 the war ended, and the size of the US Army was decreased substantially. In July of 1926 the US Army Air Corps was formed, though it was later abolished, to be succeeded by the US Army Air Force, in 1942.
In December of 1941 the Japanese attacked the US naval base at Pearl Harbor, Hawaii. This attack led to the United States, then pursuing a policy of isolationism and neutrality, into the Second World War on the side of the Allies. In the Pacific Theater, the US Army fought alongside the US Marines in an island-hopping campaign that took a number of Pacific Islands from the control of the Japanese. US Army forces also fought on the European Front. They were involved in the capture of North Africa and Sicily, and in the D-Day invasion on the beaches of Normandy that ultimately resulted in the reclamation of Europe and ultimate defeat of Nazi Germany. After the surrender of Germany in May and of Japan in August of 1945, American troops occupied both countries.
After the war ended the size of the Army again decreased. In 1947 the Air Force officially separated from the main Army in September of 1947, officially becoming the United States Air Force. In 1948 the US Army was officially desegregated. During this point the US was becoming involved in the Cold War with Russia. During this period hundreds of thousands of US soldiers were stationed in West Germany and other locations in Europe to prevent the spread of Communism, and the US Army was involved in wars against Communist troops in Korea, between 1950 and 1953, and Vietnam, between 1955 and 1975.
The Vietnam War is widely regarded as a political low point for the United States, and remains one of the least popular wars ever engaged in by the US Army. During the war the Army instituted the draft, and was severely restricted by the demands of the US Congress. Eventually political and public pressures forced the retreat, beginning in 1973 and completed in 1975.
The 1980s saw the large-scale reorganization of the US Army. It was retooled into a strictly voluntary force. It and the other four components that make up the military, the Marine Corps, Navy, Air Force and Coast Guard, were brought together under the Unified Combatant Commands. During this time the Army was involved in the 1983 invasion of Grenada and the 1989 invasion of Panama. In 1989, as the Cold War was reaching a close, the Pentagon began to plan for a reduction of Army strength. In 1990 Iraq invaded Kuwait, and Army soldiers were deployed to protect American interests in Saudi Arabia.
January of 1991 saw the beginning of Operation Desert Storm, which drove out Iraqi invaders in just 100 hours. After this, and throughout the remainder of the 1990s, the Army saw no major combat missions, though they were involved in some United Nations peacekeeping missions, such as in Somalia and Yugoslavia.
On September 11th, 2001, the United States was attacked by terrorists. The US Army, along with other parts of the American military and NATO, became involved in the Global War on Terror, invading Afghanistan in 2001, removing the ruling Taliban. In 2003 it was involved in the invasion of Iraq. There the instability of the terrain and the irregular tactics have extended the deployment time of the Army.
Currently the US Army is undergoing restructuring of command centers and changing the base unit from divisions to brigades. It has three components: the active component, also known as the Regular Army, and two reserve components, the Army National Guard and the Army Reserve.
It has ten current active divisions and several independent units. The reserves have a further eight divisions. Currently there are 549,015 active personnel, and 563,688 in the reserves.