The United States of America is a constitutional federal republic made up of 50 states and a federal district. The country is situated mostly in central North America, where its 48 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 scattered around the Caribbean and Pacific.

At 9.83 million sqkm and with over 300 million people, the United States is the third largest country 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 standards).

The nation was founded by 13 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. The Bill of Rights, comprising 10 constitutional amendments, was passed in 1791.

In the 19th 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 South and North over states' rights and the expansion of the institution of slavery started the American Civil War of the 1860s. The North's victory prevented a 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 superpower—accounting for about half of global military spending—and a dominant economic, political, and cultural force in the world.

archipelago

群岛

diverse

变化多的

immigration

移民

proclaim

宣布

amendment

修正案

split

分裂

permanent

永久的

dominant

占优势的