The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or informally America, is the most populous country in North America. It consists of 50 states, a federal district, five major unincorporated territories, nine Minor Outlying Islands, and 326 Indian reservations. It is the third-largest country by both land and total area. The United States shares land borders with Canada to its north and with Mexico to its south. It has maritime borders with the Bahamas, Cuba, Russia, and other nations. With a population of over 331 million, it is the third most populous country in the world. The national capital is Washington, D.C., and the most populous city and financial center is New York City. Paleo-aboriginals migrated from Siberia to the North American mainland at least 12,000 years ago, and advanced cultures began to appear later on. These advanced cultures had almost completely declined by the time the Europeans had arrived in North America, who subsequently began to colonize the continent. The United States emerged from the Thirteen British Colonies when disputes with the British Crown over taxation and political representation led to the American Revolution (1765–1784), which established the nation's independence. In the late 18th century, the U.S. 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 from east to west. The controversy surrounding the practice of slavery culminated in the secession of the Confederate States of America, which fought the remaining states of the Union during the American Civil War (1861–1865). With the Union's victory and preservation, slavery was abolished by the Thirteenth Amendment. By 1900, the United States had become the world's largest economy, and the Spanish–American War and World War I established the country as a world power. After Japan's surprise attack on Pearl Harbor in 1941, the U.S. entered World War II on the Allied side. The aftermath of the war left the United States and the Soviet Union as the world's two superpowers. During the Cold War, both countries engaged in a struggle for ideological dominance but avoided direct military conflict. They also competed in the Space Race, which culminated in the 1969 American spaceflight that first landed humans on the Moon. Simultaneously, the civil rights movement led to legislation abolishing state and local Jim Crow laws and other codified racial discrimination against African Americans. The Soviet Union's dissolution in 1991 ended the Cold War, leaving the United States as the world's sole superpower. The September 11 attacks in 2001 resulted in the United States launching the war on terror which included the War in Afghanistan (2001–2021) and the Iraq War (2003–2011). The United States is a federal republic with three separate branches of government, including a bicameral legislature. It is a liberal democracy and market economy; it ranks high in international measures of human rights, quality of life, income and wealth, economic competitiveness, and education; and it has low levels of perceived corruption. It has high levels of incarceration and inequality, allows capital punishment, and lacks universal health care. As a melting pot of cultures and ethnicities, the U.S. has been shaped by centuries of immigration. The United States is a highly developed country, and its economy accounts for approximately a quarter of global GDP and is the world's largest by GDP at market exchange rates. By value, the United States is the world's largest importer and second-largest exporter. Although it accounts for just over 4.2% of the world's total population, the U.S. holds over 30% of the total wealth in the world, the largest share held by any country. The United States is a founding member of the United Nations, World Bank, International Monetary Fund, Organization of American States, NATO, and is a permanent member of the United Nations Security Council. The country makes up more than a third of global military spending and is the foremost military power in the world and a leading political, cultural, and scientific force.