The United States of America is a federal presidential constitutional republic located primarily in North America, with Washington DC as its capital. The United States was founded on 4 July 1776 by the Declaration of Independence, signed by over 50 delegates from Great Britain's Thirteen Colonies in Eastern North America. The United States won its independence from the British Empire during the American Revolutionary War, which ended with the Treaty of Paris on 3 September 1783. The confederation of states which emerged from the Revolutionary War evolved into a federal republic with the adoption of the US Constitution on 21 June 1788, after which the United States would go on to expand westward through both violent (the Northwest Indian War, Mexican-American War, and Plains Indian Wars) and peaceful (Louisiana Purchase, Adams-Onis Treaty, and Alaska Purchase) means, admitting new states after displacing their Native American populations and replacing them with (principally white) settlers and immigrants. Slavery was legal in the United States until the American Civil War of 1861-1865, which resulted in the emancipation of the enslaved African-Americans in the American South; however, following the end of Reconstruction in 1877, white supremacist Southern state governments passed several discriminatory laws to segregate and economically marginalize Black communities, laws which would not be found unconstitutional and removed until the 1950s and 1960s, and whose legacies survive in the form of continued economic and social disparities between African-Americans and white Americans.
The United States became an imperial power after its acquisition of Hawaii in 1893 and the Spanish-American War in 1898 (which gave the United States control of several Caribbean and Pacific islands, of which Puerto Rico and Guam, among others, are still American possessions), and the United States became a global power following its belated entry into World War I in 1917, after which the country continued to play a major role as an arbiter of international affairs. While the United States briefly retreated into isolationism after World War I, its (again belated) involvement in World War II transformed the United States into a superpower and the de facto leader of the "free world"; the United States co-founded the United Nations, NATO, the World Bank, the International Monetary Fund, and the Organization of American States during this time. The US-led Western Bloc would go on to engage in a decades-long standoff with the Soviet-led Eastern Bloc in the Cold War from 1945 to 1991, during which time the United States acquired a global image as the chief capitalist and democratic power in the world; the Dissolution of the Soviet Union and the end of the Cold War in 1991 were seen as the final victory of American liberal democracy and the birth of a "unipolar world" dominated by an American-led "New World Order". By the 21st century, America ranked highly in international measures of economic freedom, quality of life, education, and human rights, and was known as a diverse "melting pot" and a "nation of immigrants".
However, America lagged behind other major nations in racial, wealth, and income equality, as well as controversially retaining capital punishment, mass incarceration, and a lack of universal healthcare. By 2021, the United States had an estimated population of 331,893,745 people, of whom 61.6% were white, 12.4% Black, 6% Asian, 1.1% Native American, .2% Pacific Islander, 10.2% multiracial, and 8.4% others; 18.7% of Americans were Hispanic. By 2021, the country was 63% Christian (40% Protestant, 21% Catholic, and 2% other), 29% irreligious, 6% others, and 2% unanswered.