The United States is a federal constitutional republic in North America made up of 50 states and Washington, D.C., with additional island territories in the Caribbean and Oceania. Indigenous peoples lived across the continent for thousands of years before European exploration and colonization began after 1492. The nation emerged from the imperial crisis with Britain, declared independence in 1776, and gained international recognition after the Revolutionary War in 1783. It expanded westward during the 19th century, while conflict over slavery led to the Civil War and abolition. By 1900 it had become a great power, and in the 20th century it grew into a dominant global political, economic, military, and cultural force.