The United States of America (USA or U.S.A.), commonly referred to as the United States (US or U.S.), America, or simply the States, is a federal republic[10][11] consisting of 50 states and a federal district. The 48 contiguous states and the federal district of Washington, D.C. are in central North America between Canada and Mexico. The state of Alaska is the northwestern part of North America, west of Canada and east of Russia which is across the Bering Strait in Asia, and the state of Hawaii is an archipelago in the mid-North Pacific. The country also has five populated and nine unpopulated territories in the Pacific and the Caribbean. The U.S. exercises full international defense authority and responsibility for three sovereign nations through Compact of Free Association with Micronesia, the Marshall Islands and Palau, all of which are Pacific island nations which became US territories after World War II and gained independence in subsequent years.

At 3.79 million square miles (9.83 million km2) in total and with around 316 million people, the United States is the fourth-largest in total area, the fifth largest in the contiguous area and third in population. It is one of the world's most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries.[12] 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 wildlife.

Paleo-indians migrated from Asia to what is now the United States mainland around 12,000 years ago. European colonization began around 1600, mostly from England. The United States emerged from 13 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.[13][14] The current Constitution was adopted on September 17, 1787; 27 Amendments have since been added to the Constitution. The first 10 amendments, collectively named the Bill of Rights, were ratified in 1791 and guarantee many fundamental civil rights and freedoms.

Driven by the doctrine of manifest destiny, the United States embarked on a vigorous expansion across North America throughout the 19th century.[15] This involved displacing native tribes, acquiring new territories, and gradually admitting new