The United States of America is a federal republic in North America, comprising 50 states, the capital district of Washington, D.C., and several island territories, with a population exceeding 341 million and the world's fourth-largest land area. Its history traces back to Paleo-Indians who arrived around 15,000 years ago, followed by British colonization beginning with Virginia in 1607; tensions over taxation and rights sparked the American Revolution, leading to the formal declaration of independence on July 4, 1776. After westward expansion, the nation was torn apart by the Civil War (1861–1865), which ended slavery and reunified the country, before it emerged as a global power through World Wars I and II, ultimately becoming the world's sole superpower after the Soviet Union's collapse in 1991. Governed by the U.S. Constitution, the country features a separation of powers among the presidency, a bicameral Congress (House and Senate), and an independent judiciary, with significant autonomy granted to the 50 states. As a developed nation, it has boasted the world's largest economy since about 1890, ranking high in innovation and higher education, yet facing pronounced wealth inequality. Additionally, the U.S. accounts for nearly a third of global military spending, is widely seen as having the most powerful armed forces, and exerts profound influence over global politics, culture, and economics.