The United States Army is the branch of the United States armed forces responsible for land-based military operations. It is the largest and oldest established branch of the U.S. military and is one of seven uniformed services. The modern Army has its roots in the Continental Army which was formed on 14 June 1775, before the establishment of the United States, to meet the demands of the American Revolutionary War. Congress created the United States Army on 14 June 1784 after the end of the war to replace the disbanded Continental Army. The Army considers itself to be descended from the Continental Army and thus dates its inception from the origins of that force.
The primary mission of the Army is to "provide necessary forces and capabilities ... in support of the National Security and Defense Strategies."Control and operation is administered by the Department of the Army, one of the three service departments of the Department of Defense. The civilian head is the Secretary of the Army and the highest ranking military officer in the department is the Chief of Staff, unless the Chairman of the Joint Chiefs of Staff or Vice Chairman of the Joint Chiefs of Staff are Army officers. The Regular Army reported a strength of 539,675 soldiers; the Army National Guard (ARNG) reported 360,351 and the United States Army Reserve (USAR) reported 197,024 putting the combined component strength total 1,097,050 soldiers (2008 Financial).

Origins
The Continental Army was created on 14 June 1775 by the Continental Congress as a unified army for the states to fight Great Britain, with George Washington appointed as its commander.The Army was initially led by men who had served in the British Army or colonial militias and who brought much of British military heritage with them. As the Revolutionary war progressed, French aid, resources, and military thinking influenced the new army, while Prussian assistance and instructors also had a strong influence.
George Washington made use of the Fabian strategy and used hit-and-run tactics, hitting where the enemy was weakest, to wear down the British forces and their Hessian mercenary allies. Washington led victories against the British at Trenton and Princeton, and then turned south. With a decisive victory at Yorktown, and the help of the French, the Spanish and the Dutch, the Continental Army prevailed against the British, and with the Treaty of Paris, the independence of the United States was acknowledged.
After the war, though, the Continental Army was quickly disbanded as part of the American distrust of standing armies, and irregular state militias became the new nation's sole ground army, with the exception of a regiment to guard the Western Frontier and one battery of artillery guarding West Point's arsenal. However, because of continuing conflict with Native Americans, it was soon realized that it was necessary to field a trained standing army. The first of these, the Legion of the United States, was established in 1791.
19th century
The War of 1812 (1812-1815), the second and last American war against the British, was less successful than the Revolution had been. An invasion of Canada failed, and U.S. troops were unable to stop the British from burning the new capital of Washington, D.C.. However, the Regular Army, under Generals Winfield Scott and Jacob Brown, proved they were professional and capable of defeating a British army in the Niagara campaign of 1814. Two weeks after a treaty was signed, though, Andrew Jackson defeated the British invasion of New Orleans. However this had little effect, as per the treaty both sides returned to the status quo.
Between 1815 and 1860, a spirit of Manifest Destiny struck the United States, and as settlers moved west the U.S. Army engaged in a long series of skirmishes and battles with Native Americans that the colonists uprooted. The U.S. Army also fought the short Mexican–American War, which was a victory for the United States and resulted in territory which became all or parts of the states of California, Nevada, Utah, Colorado, Arizona, Wyoming and New Mexico.
The Civil War (1861-1865) was the most costly war for the United States. After most states in the South seceded to form the Confederate States of America, CSA troops opened fire on the Union-held Fort Sumter in Charleston, South Carolina, starting the war. For the first two years Confederate forces solidly defeated the U.S. Army, but after the decisive battles of Gettysburg in the east and Vicksburg in the west, combined with superior industrial might and numbers, Union troops fought a brutal campaign through Confederate territory and the war ended with a Confederate surrender at Appomatox Courthouse in April 1865. Based on 1860 census figures, 8% of all white males aged 13 to 43 died in the war, including 6% in the North and an extraordinary 18% in the South.
Following the Civil War, the U.S. Army fought a long battle with Native Americans, who resisted U.S. expansion into the center of the continent. But by the 1890s the U.S. saw itself as a potential international player. U.S. victories in the Spanish-American War (1898) and the controversial and less well known Philippine-American War (1898-1913), as well as U.S. intervention in Latin America and the Boxer Rebellion, gained America more land.

20th century
The United States joined World War I (1914-1918) in 1917 on the side of Russia, Britain and France. U.S. troops were sent to the front and were involved in the push that finally broke through the German lines. With the armistice on 11 November 1918, the Army once again decreased its forces.
The U.S. joined World War II after the Japanese attack on Pearl Harbor on 7 December 1941. On the European front, U.S. Army troops formed a significant portion of the forces that captured North Africa and Sicily. On D-Day and in the subsequent liberation of Europe and defeat of Germany, millions of U.S. Army troops played a central role. In the Pacific, Army soldiers participated alongside U.S. Marines in the "island hopping" campaign that wrested the Pacific Islands from Japanese control. Following the Axis surrenders in May (Germany) and September (Japan) of 1945, Army troops were deployed to Japan and Germany to occupy the two defeated nations. Two years after World War II, the Army Air Forces separated from the Army to become the United States Air Force on 18 September 1947 after decades of attempting to separate. Also, in 1948 the Army was desegregated.
However, the end of World War II set the stage for the East-West confrontation known as the Cold War (late 1940s to late 1980s/early 1990s). With the outbreak of the Korean War, concerns over the defence of Western Europe rose. Two corps, V and VII, were reactivated under Seventh United States Army in 1950 and American strength in Europe rose from one division to four. Hundreds of thousands of U.S. troops remained stationed in West Germany, with others in Belgium, the Netherlands and the United Kingdom, until the 1990s in anticipation of a possible Soviet attack.
During the Cold War, American troops and their allies fought Communist forces in Korea and Vietnam (see Domino Theory). The Korean War began in 1950, when the Soviets walked out of a U. N. Security meeting, removing their possible veto. Under a United Nations umbrella, hundreds of thousands of U.S. troops fought to prevent the takeover of South Korea by North Korea, and later, to invade the northern nation. After repeated advances and retreats by both sides, and the Peoples' Republic of China 's entry into the war, a cease-fire returned the peninsula to the status quo in 1953.
The Vietnam War is often regarded as a low point in the Army's record due to the use of drafted personnel, the unpopularity of the war with the American public, and frustrating restrictions placed on the Army by US political leaders (i.e. no invading communist held North Vietnam). While American forces had been stationed in the Republic of Vietnam since 1959, in intelligence & advising/training roles, they did not deploy in large numbers until 1965, after the Gulf of Tonkin Incident. American forces effectively established and maintained control of the "traditional" battlefield, however they struggled to counter the guerrilla hit and run tactics of the communist Viet Cong and the North Vietnamese Army. On a tactical level, American soldiers (and the US military as a whole) never lost a sizable battle.[citation needed] For instance in the Tet Offensive in 1968, the US Army turned a large scale attack by communist forces into a massive defeat of the Viet Cong on the battlefield (though at the time the offensive sapped the political will of the American public) which permanently weakened the guerrilla force; thereafter, most large scale engagements were fought with the regular North Vietnamese Army. In 1973 domestic political opposition to the war finally forced a US withdrawal. In 1975, Vietnam was unified under a communist government.
The Total Force Policy was adopted by Chief of Staff of the Army General Creighton Abrams in the aftermath of the Vietnam War and involves treating the three components of the Army - the Regular Army, the Army National Guard and the Army Reserve as a single force. Believing that no US president should be able to take the United States (and more specifically the US Army) to war without the support of the American people, General Abrams intertwined the structure of the three components of the Army in such a way as to make extended operations impossible, without the involvement of both the Army National Guard and the Army Reserve.
The 1980s was mostly a decade of reorganization. The Army converted to an all-volunteer force with greater emphasis on training and technology. The Goldwater-Nichols Act of 1986 created Unified Combatant Commands bringing the Army together with the other four armed forces under unified, geographically organized command structures. The Army also played a role in the invasions of Grenada in 1983 (Operation Urgent Fury) and Panama in 1989 (Operation Just Cause).
By 1991 Germany was reunited and the Soviet Union was near collapse. The Cold War was, effectively, over. In 1990 Iraq invaded its smaller neighbor, Kuwait. In January 1991 Operation Desert Storm commenced, a U.S.-led coalition which deployed over 500,000 troops, the bulk of them from U.S. Army formations, to drive out Iraqi forces. The campaign ended in total victory for the Army, as Western coalition forces routed an Iraqi Army organized along Soviet lines in just one hundred hours.
After Desert Storm, the Army did not see major combat operations for the remainder of the 1990s. Army units did participate in a number of peacekeeping activities, such as the UN peacekeeping mission in Somalia in 1993, where the abortive Operation Gothic Serpent led to the deaths of eighteen American soldiers and the withdrawal of international forces. The Army also contributed troops to a NATO peacekeeping force in the former Yugoslavia in the middle of the decade.

21st century
After the September 11 attacks, and as part of the Global War on Terror, U.S. and NATO combined arms (i.e. Army, Navy, Air Force, Marine, Special Operations) forces invaded Afghanistan in 2001, replacing the Taliban government.
The Army took part in the combined U.S. and allied invasion of Iraq in 2003. In the following years the mission changed from conflict between regular militaries to counterinsurgency, with large numbers of suicide attacks resulting in the deaths of more than 4,000 U.S. servicemen (as of March 2008) and injuries to thousands more.[7] The lack of stability in the theater of operations has led to longer deployments for Regular Army as well as Reserve and Guard troops.