The United States of America (USA), commonly referred to as the United States (U.S.) or America, is a federal republic consisting of 50 states and a federal district. The 48 contiguous states and Washington, D.C., are in central North America between Canada and Mexico. The state of Alaska is located in the northwestern part of North America and the state of Hawaii is an archipelago in the mid-Pacific. The country also has five populated and numerous unpopulated territories in the Pacific and the Caribbean. At 3.80 million square miles (9.85 million km2) and with over 320 million people, the United States is the world's fourth-largest country by total area and third most populous. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries.The geography and climate of the United States are also extremely diverse, and the country is home to a wide variety of wildlife.
Paleo-Indians migrated from Eurasia to what is now the U.S. mainland around 15,000 years ago,with European colonization beginning in the 16th century. The United States emerged from 13 British colonies located along the East Coast. Disputes between Great Britain and the colonies led to the American Revolution. On July 4, 1776, as the colonies were fighting Great Britain in the American Revolutionary War, delegates from the 13 colonies unanimously adopted the Declaration of Independence. The war ended in 1783 with recognition of the independence of the United States by the Kingdom of Great Britain, and was the first successful war of independence against a European colonial empire.The country's constitution was adopted on September 17, 1787 and ratified by the states in 1788. The first ten amendments, collectively named the Bill of Rights, were ratified in 1791 and designed to guarantee many fundamental civil rights and freedoms.