The American Revolution was the political upheaval during the last half of the 18th century in which thirteen colonies in North America joined together to break free of the British Empire, combining to become the United States of America. The Revolution includes political, social, and military aspects. The Revolutionary era is generally considered to have begun with the Battles of Lexington and Concord (1775) and ended with the ratification of the United States Constitution in 1789. The military phase of the Revolution, the American Revolutionary War, lasted from 1775 to 1783.