第3个回答 2006-03-22
The United States of America is a constitutional federal republic, situated primarily in North America. It comprises 50 states and one federal district, and has several territories with differing degrees of affiliation. It is also referred to, with varying formality, as the United States, the U.S., the U.S.A., the U.S. of A., America[1], the States, or (poetically) Columbia.
Since the mid-20th century, following World War II, the United States has emerged as a dominant global influence in economic, political, military, scientific, technological, and cultural affairs. Because of its influence, the U.S. is considered a superpower and, particularly after the Cold War, a hyperpower by some.
The country celebrates its founding date as July 4, 1776, when the Second Continental Congress—representing thirteen British colonies—adopted the Declaration of Independence that rejected British authority in favor of self-determination.
The structure of the government was profoundly changed in 1789, when the states replaced the Articles of Confederation with the United States Constitution. The date on which each of the fifty states adopted the Constitution is typically regarded as the date that state "entered the Union" to become part of the United States.