The United States of America, commonly known as the United States (U.S.) or America, is a federal republic composed of 50 states, a federal district, five major self-governing territories, and various possessions. Forty-eight of the fifty states and the federal district are contiguous and located in North America between Canada and Mexico. The state of Alaska is in the northwest corner of North America, bordered by Canada to the east and across the Bering Strait from Russia to the west. The state of Hawaii is an archipelago in the mid-Pacific Ocean. The U.S. territories are scattered about the Pacific Ocean and the Caribbean Sea and encompass nine time zones. The extremely diverse geography, climate and wildlife of the United States make it one of the world's 17 megadiverse countries. At 3.8 million square miles (9.8 million km2) and with over 324 million people, the United States is the world's third- or fourth-largest country by total area, third-largest by land area, and the third-most populous. It is one of the world's most ethnically diverse and multicultural nations, and is home to the world's largest immigrant population. The capital is Washington, D.C., and the largest city is New York City; nine other major metropolitan areas—each with at least 4.5 million inhabitants—are Los Angeles, Chicago, Dallas, Houston, Philadelphia, Miami, Atlanta, Boston, and San Francisco. Paleo-Indians migrated from Asia to the North American mainland at least 15,000 years ago. European colonization began in the 16th century. The United States emerged from 13 British colonies along the East Coast. Numerous disputes between Great Britain and the colonies following the Seven Years' War led to the American Revolution, which began in 1775. On July 4, 1776, during the course of the American Revolutionary War, the colonies unanimously adopted the Declaration of Independence. The war ended in 1783 with recognition of the independence of the United States by Great Britain, representing the first successful war of independence against a European power. The current constitution was adopted in 1788, after the Articles of Confederation, adopted in 1781, were felt to have provided inadequate federal powers. The first ten amendments, collectively named the Bill of Rights, were ratified in 1791 and designed to guarantee many fundamental civil liberties.The United States embarked on a vigorous expansion across North America throughout the 19th century,[31] displacing Native American tribes, acquiring new territories, and gradually admitting new states until it spanned the continent by 1848.[31] During the second half of the 19th century, the American Civil War led to the end of legal slavery in the country. By the end of that century, the United States extended into the Pacific Ocean, and its 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 a permanent member of the United Nations Security Council. The end of the Cold War and the dissolution of the Soviet Union in 1991 left the United States as the world's sole superpower. The U.S. is a founding member of the United Nations, World Bank, International Monetary Fund, Organization of American States (OAS), and other international organizations. The United States is a highly developed country, with the world's largest economy by nominal GDP and second-largest economy by PPP. Though its population is only 4.3% of the world total,[37] Americans hold nearly 40% of the total wealth in the world.[38] The United States ranks among the highest in several measures of socioeconomic performance, including average wage,[39] human development, per capita GDP, and productivity per person.[40] While the U.S. economy is considered post-industrial, characterized by the dominance of services and knowledge economy, the manufacturing sector remains the second-largest in the world.[41] Accounting for approximately a quarter of global GDP[42] and a third of global military spending,[43] the United States is the world's foremost economic and military power. The United States is a prominent political and cultural force internationally, and a leader in scientific research and technological innovations.