Declaration of Independence in the year 1776, yet its territory was occupied first by the Native Americans since prehistoric times and then also by European colonists mostly following the voyages of Christopher Columbus starting in 1492. The Thirteen Colonies declared independence from the British Empire during the American Revolution and as states ratified the Articles of Confederation. In 1789 the Constitution became the basis for the United States federal government. The young nation continued to struggle with the scope of central government and with European influence, creating the first political parties in the 1790s, and fighting a second war for independence in 1812.
U.S. territory expanded westward across the continent, brushing aside Native Americans and Mexico, and overcoming modernizers who wanted to deepen the economy rather than expand the geography. Slavery of Africans was abolished in the North, but heavy world demand for cotton let it flourish in the Southern states. The 1860 election of Abraham Lincoln calling for no more expansion of slavery triggered a crisis as eleven slave states seceded to found the Confederate States of America in 1861. The bloody American Civil War (1861–65) redefined the nation and remains the central iconic event. The South was defeated and, in the Reconstruction era, the U.S. ended slavery, extended rights to African Americans, and readmitted secessionist states with loyal governments. The present 48 contiguous states were admitted by early 1912.
The U.S. rose as an industrialized power by the early 20th century. Lifestyle changes led to the Progressive movement, which pushed for reform in industry and politics and is associated with women's suffrage and Prohibition of alcohol (the latter failed by 1933). Initially neutral in World War I, the U.S. eventually declared war on Germany in 1917, yet popular support for non-interventionism derailed post-war attempts to foster international cooperation. The Wall Street Crash of 1929 punctuated the onset of the Great Depression, to which the federal government responded with New Deal recovery programs. After the Japanese attack on Pearl Harbor in December 1941, the U.S. entered World War II alongside the Allies and helped defeat Nazi Germany in Europe and, with the detonation of newly-invented atomic bombs, Japan in Asia and the Pacific.
The Soviet Union and the U.S. emerged as opposing superpowers after the war and began the Cold War confronting indirectly in an arms race, the Space Race, and intervention in Europe and eastern Asia.
Liberalism reflected in the civil rights movement and opposition to war in Vietnam peaked in the 1960s–70s before giving way to conservatism in the early 1980s. The Cold War ended when the Soviet Union dissolved in 1991, leaving the U.S. to prosper in the booming Information Age economy that was boosted, at least in part, by information technology. International conflict and economic uncertainty heightened by 2001 with the September 11 attacks and subsequent War on Terror and the late-2000s recession.