The United States of America (USA or U.S.A.), commonly called the United States (US or U.S.) or America, is a federal republic[10][11] consisting of fifty states, a federal district and territories.[12] The lower forty-eight contiguous states and the federal district of Washington, D.C. are in central North America between Canada and Mexico. The state of Alaska is west of Canada and east of Russia across the Bering Strait, and the state of Hawaii is in the mid-North Pacific. The five populated and nine unpopulated territories lie in the Pacific and the Caribbean.
At 3.79 million square miles (9.83 million km2) and with around 315 million people, the United States is the third- or fourth-largest country by total area and the third-largest by both land area and population. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries.[13] The geography and climate of the U.S. is also extremely diverse, with deserts, plains, forests, and mountains that are home to a wide variety of species.
Paleoindians migrated from Asia to what is now the United States mainland around 15,000 years ago. After 1500, Old World diseases introduced by Europeans greatly reduced their populations. European colonization began around 1600 and came mostly from England. The United States emerged from thirteen British colonies located along the Atlantic seaboard. Disputes between Great Britain and the American colonies led to the American Revolution. On July 4, 1776, delegates from the 13 colonies unanimously issued the Declaration of Independence, which established the United States of America. The American Revolutionary War, which ended with the recognition of independence of the United States from the Kingdom of Great Britain, was the first successful war of independence against a European colonial empire.[14][15] The current Constitution was adopted on September 17, 1787; twenty-seven Amendments have since been...