The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territories, 326 Indian reservations, and some minor possessions. At 3.8 million square miles (9.8 million square kilometers), it is the world's third- or fourth-largest country by total area. The United States shares significant land borders with Canada to the north and Mexico to the south, as well as limited maritime borders with the Bahamas, Cuba, and Russia. With a population of more than 331 million people, it is the third most populous country in the world. The national capital is Washington, D.C., and the most populous city is New York City.

Paleo-Indians migrated from Siberia to the North American mainland at least 12,000 years ago, and European colonization began in the 16th century. The United States emerged from the thirteen British colonies established along the East Coast. Disputes over taxation and political representation with Great Britain led to the American Revolutionary War (1775–1783). In 1788, four years after achieving independence, the states ratified the U.S. Constitution, establishing a new federal government which remains in force today. In the early 19th century, the country began expanding across North America, gradually obtaining new territories, sometimes through war, frequently displacing Native Americans, and admitting new states; by 1848, the United States spanned the continent. Slavery was legal in the southern states, which sparked the American Civil War (1861–1865). The ultimate victory by the Union led to slavery's abolition. The war and its aftermath saw the rapid expansion of American industrial capabilities. A more interventionist American foreign policy was confirmed by the overseas territories won by the U.S. in the Spanish–American War. Victory in the First World War in 1918 made the United States into a world power. The U.S. forged a strong new cultural identity in the 1920s and 1930s, with the popularization of baseball and the growing international attraction of Hollywood and jazz.

In 1941, the United States formally entered the Second World War as a member of the Allied powers after the surprise attack on Pearl Harbor. Its armed forces fought simultaneously in two military theaters, Western Europe and the Pacific War. The country also experienced an unparalleled transformation that saw the rapid expansion of its military, scientific, and industrial might. It also pursued the Manhattan Project, a top secret effort to develop the atomic bomb. The United States came out of the war as a superpower and the only nation with nuclear weapons. It played a significant role in establishing the United Nations and drafting the 1947 Constitution of Japan, which had been defeated in 1945.

In the aftermath of the Second World War, the United States and the Soviet Union faced growing tensions that escalated into the Cold War, which would play out for the duration of the 20th century. It also posed the potential of nuclear conflict when in 1949 the Soviet Union detonated its first atomic weapon. The United States opposed Soviet allies in the Korean War (with a UN mandate) and the Vietnam War (without UN support) but avoided direct military conflict with the Soviet Union itself. The Space Race, another measure of U.S.–Soviet competition during the Cold War, resulted in the Apollo 11 mission that saw the United States land the first man on the Moon in 1969. The dissolution of the Soviet Union in 1991 ended the Cold War, leaving the United States as the world's sole superpower for the next two decades. In the 21st century, the United States has increasingly been challenged by China as a dominant superpower.

The United States is a federal republic and a representative democracy with three separate branches of government, including a bicameral legislature. It is a founding member of the United Nations, World Bank, International Monetary Fund, Organization of American States, NATO, and other international organizations. It is a permanent member of the United Nations Security Council. Considered a melting pot of cultures and ethnicities, its population has been profoundly shaped by centuries of immigration. The country ranks high in international measures of economic freedom, quality of life, education, and human rights, and has low levels of perceived corruption. However, the country has received domestic and international criticism concerning inequality related to race, wealth and income, the use of capital punishment, high incarceration rates, and lack of universal health care.

The United States is a highly developed country, accounts for approximately a quarter of global GDP, and is the world's largest economy. By value, the United States is the world's largest importer and the second-largest exporter of goods. Although its population is only 4.2% of the world's total, it holds 29.4% of the total wealth in the world, the largest share held by any country. Making up more than a third of global military spending, it is the foremost military power in the world; and it is a leading political, cultural, and scientific force internationally.

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