The United States of America, colloquially known as the United States, the USA, the US, or simply America; is a sovereign state located in North America. The United States is bordered by Canada to the north and Mexico to the south. Originally a collection of British colonies along the Atlantic Ocean, the US declared their independence on July 4, 1776, resulting in the American Revolutionary War. During the early 1880s, the US expanded to the Pacific Ocean and beyond. Debate over the expansion of slavery into these new territories sparked the American Civil War between 1861 until 1865, becoming one of the bloodiest wars during the 19th Century.
By the 1900s, the United States developed itself into a great power. The US attempted to remain neutral during both World War I and World War II, but eventually got involved due to tensions from its allies. The Japanese attack at Pearl Harbor in 1941 became the catalyst for the US' involvement in WWII. Both the United States and the Soviet Union emerged from the war as the two superpowers. With both pushing for global superiority, the period between 1945 until 1990 was known as the Cold War, a time of proxy wars and the threat of nuclear war. Following the democratization of the USSR in the 1990s, relations between the superpowers has been peaceful (though a rivalry continues in many respects). Following the attacks on September 11, 2001, the United States became involved in a new War on Terror. This includes the 2001 invasion of Sudan, which allowed a safe haven for terrorist groups.
The United States is a federation of 52 states. The two most recent states (Puerto Rico and Mariana) were admitted into the union in 2001 and 2003, respectively. The District of Columbia (which includes the capital of Washington) is a territory under the control of the federal government. Along with the states and D.C., the United States also controls several island territories in both the Atlantic and the Pacific (the largest of which is American Samoa).