Originally part of the British Empire, the Thirteen Colonies declared independence in 1776, later to establish a federation between them. From their original claims, the US expanded to purchase Louisiana from France in 1804, annex Upper Canada in 1845, annex several islands in the Caribbean during the later-half of the 19th century, and annexing Panama in the early 1900s.
After World War II, the United States emerged as one of the predominant superpowers (along with the Soviet Union). Today, the US has one of the largest economies and armed forces of the world.
The United States is a federation made up of 45 states spanning from the continental 41 to the four in the Caribbean Sea. The US also includes several unincorporated territories (mostly in the Pacific and the Arctic).