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The United States of America is a constitutional federal republic comprising fifty states and a federal district. The country is situated mostly in central North America, where its forty-eight contiguous states and Washington, D.C., the capital district, lie 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 Russia to the west across the Bering Strait, and the state of Hawaii is an archipelago in the mid-Pacific. The United States also possesses several territories, or insular areas, scattered around the Caribbean and Pacific.

At 3.79 million square miles (9.83 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 by population. The United States is one of the world's most ethnically diverse nations, the product of large-scale immigration from many countries. The U.S. economy is the largest national economy in the world, with a nominal 2006 gross domestic product (GDP) of more than US$13 trillion (over 19% of the world total based on purchasing power parity).

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 Great 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, Great Britain, Mexico, and Russia, and annexed the Republic of Texas and the Republic of Hawaii. Disputes between the agrarian South and industrial North over states' rights and the expansion of the institution of slavery provoked the American Civil War of the 1860s. The North's victory prevented a permanent split of the country and led to the end of slavery in the United States. The Spanish-American War and World War I confirmed the nation's status as a military power. In 1945, the United States emerged from World War II as the first country with nuclear weapons, a permanent member of the United Nations Security Council, and a founding member of NATO. In the post-Cold War era, the United States is the only remaining superpoweraccounting for approximately 50% of global military spendingand a dominant economic, political, and cultural force in the world.

The coastal plain of the Atlantic seaboard gives way further inland to deciduous forests and the rolling hills of the Piedmont. The Appalachian Mountains divide the eastern seaboard from the Great Lakes and the grasslands of the Midwest. The Mississippi-Missouri River, the world's fourth longest river system, runs mainly north-south through the heart of the country. The flat, fertile prairie land of the Great Plains stretches to the west. The Rocky Mountains, at the western edge of the Great Plains, extend north to south across the continental United States, reaching altitudes higher than 14,000 feet (4,300 m) in Colorado. The area to the west of the Rocky Mountains is dominated by the rocky Great Basin and deserts such as the Mojave. The Sierra Nevada range runs parallel to the Rockies, relatively close to the Pacific coast. At 20,320 feet (6,194 m), Alaska's Mount McKinley is the country's tallest peak. Active volcanoes are common throughout the Alexander and Aleutian Islands, and the entire state of Hawaii is built upon tropical volcanic islands. The super volcano underlying Yellowstone National Park in the Rockies is the continent's largest volcanic feature.

Because of the United States' large size and wide range of geographic features, nearly every type of climate is represented. The climate is temperate in most areas, tropical in Hawaii and southern Florida, polar in Alaska, semi-arid in the Great Plains west of the 100th meridian, desert in the Southwest, Mediterranean in Coastal California, and arid in the Great Basin. Extreme weather is not uncommonthe states bordering the Gulf of Mexico are prone to hurricanes, and most of the world's tornadoes occur within the continental United States, primarily in the Midwest.