The United States of America is a federal constitutional republic comprising fifty states, one federal district, and fourteen territories. The country is situated almost entirely in the western hemisphere: its forty-eight contiguous states and Washington, D.C., the capital district, lie in central North America between the Pacific and Atlantic Oceans, bordered by Canada to the north and Mexico to the south; the state of Alaska is in the northwest of the continent with Canada to its east, and the state of Hawaii is in the mid-Pacific. U.S. territories, or insular areas, are scattered around the Caribbean and Pacific.
At 3.7 million square miles (9.6 million km²) and with over 300 million people, the United States is the third or fourth largest country by total area, and third largest by land area and population. The United States is one of the world's most ethnically diverse nations, the product of large-scale immigration from many countries. Its national economy is the largest in the world, with a nominal 2006 gross domestic product (GDP) of more than US$13 trillion.
The nation was founded by thirteen colonies of Great Britain located along the Atlantic seaboard. Proclaiming themselves "states," they issued the Declaration of Independence on July 4, 1776. The rebellious states defeated Britain in the American Revolutionary War, the first successful colonial war of independence. A federal convention adopted the current United States Constitution on September 17, 1787; its ratification the following year made the states part of a single republic. The Bill of Rights, comprising ten constitutional amendments, was ratified in 1791. In the nineteenth century, the United States acquired land from France, Spain, Mexico, and Russia, and annexed the Republic of Texas and the Republic of Hawaii. The American Civil War ended slavery in the United States and prevented a permanent split of the country. The Spanish-American War and World War I confirmed its status as a military power. In 1945, the United States emerged from World War II as the first country with nuclear weapons and a permanent member of the United Nations Security Council. The sole remaining superpower in the post–Cold War era, the United States is perceived by many as the dominant economic, political, cultural, and military force in the world.
