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 nine 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-largest by population. 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 issued the Declaration of Independence. The war ended in 1783 with the recognition of independence of the United States from the Kingdom of Great Britain, and was the first successful war of independence against a European colonial empire. The United States is the only country that ever won its independence from Great Britain by war. 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. Driven by the doctrine of manifest destiny, the United States embarked on a vigorous expansion across North America throughout the 19th century. This involved displacing native tribes, acquiring new territories, and gradually admitting new states, until by 1848 the nation spanned the continent. During the second half of the 19th century, the American Civil War ended legal slavery in the country. By the end of that century, the United States extended into the Pacific Ocean, and the economy, driven in large part by the Industrial Revolution, began to soar. The Spanish–American War and World War I confirmed the country's status as a global military power. The United States emerged from World War II as a global superpower, the first country to develop nuclear weapons, the only country to use them in warfare, and as a permanent member of the United Nations Security Council. The end of the Cold War and the dissolution of the Soviet Union left the United States as the sole superpower. The United States is a developed country and has the world's largest national economy, benefiting from an abundance of natural resources and high worker productivity. While the U.S. economy is considered post-industrial, the country continues to be one of the world's largest manufacturers. Accounting for 37% of global military spending, it is the world's foremost economic and military power, a prominent political and cultural force, and a leader in scientific research and technological innovations.