The first known inhabitants of modern-day United States territory are believed to have arrived over a period of several thousand years beginning sometime prior to 15,000 - 50,000 years ago by crossing Beringia into Alaska. Solid evidence of these cultures settling in what would become the US is dated to around 14,000 years ago.

Research has revealed much about the early Native American settlers of North America. Columbus' men were the first documented Old Worlders to land in the territory of what is now the United States when they arrived in Puerto Rico during their second voyage in 1493. Juan Ponce de León, who arrived in Florida in 1513, is credited as being the first European to land in what is now the continental United States, although some evidence suggests that John Cabot might have reached what is presently New England in 1498.

In its beginnings, the United States of America consisted only of the Thirteen Colonies, which consisted of states occupying the same lands as when they were British colonies. American colonists fought off the British army in the American Revolutionary War of the 1770s and issued a Declaration of Independence in 1776. Seven years later, the signing of the Treaty of Paris officially recognized independence from Britain. In the nineteenth century, westward expansion of United States territory began, upon the belief of Manifest Destiny, in which the United States would occupy all the North American land east to west, from the Atlantic to the Pacific Oceans. By 1912, with the admission of Arizona to the Union, the U.S. reached that goal. The outlying states of Alaska and Hawaii were both admitted in 1959.

Ratified in 1788, the Constitution serves as the supreme American law in organizing the government; the Supreme Court is responsible for upholding Constitutional law. Many forms of social progress started in the nineteenth century; those advancements have been widely reflected in the Constitution. Slavery was abolished in 1865 by the Thirteenth Amendment to the United States Constitution; the following Fourteenth and Fifteenth Amendments respectively guaranteed citizenship for all persons naturalized within U.S. territory and voting for people of all races. In later years, civil rights were extended to women and black Americans, following effective lobbying from social activists. The Nineteenth Amendment prohibited gender discrimination in voting rights; later, the Civil Rights Act of 1964 outlawed racial segregation in public places.

The Progressive Era marked a time of economic growth for the United States, advancing to the Roaring Twenties. However, the Wall Street Crash of 1929 led to the Great Depression, a time of economic downturn and mass unemployment. Consequently, the U.S. government established the New Deal, a series of reform programs that intended to assist those affected by the Depression. The New Deal had varied success. However, once the U.S. entered World War II in December 1941, the economy quickly recovered, so much that the U.S. became a world superpower by the dawn of the Cold War. During the Cold War, the U.S. and the Soviet Union were the world's two superpowers, but with the end of the Cold War and the collapse of the Soviet Union, United States became the world's only superpower.