The United States of America was formed after thirteen British colonies in North America declared independence from the British Empire on July 4, 1776. In the Lee Resolution, passed by the Second Continental Congress two days prior, the colonies resolved that they were free and independent states. The union was formalized in the Articles of Confederation, which came into force on March 1, 1781, after being ratified by all 13 states. Their independence was recognized by Great Britain in the Treaty of Paris of 1783, which concluded the American Revolutionary War. This extended the borders of the new nation westward past the Proclamation Line to the Mississippi River. This land was organized into territories and then states, though there remained some conflict with the sea-to-sea grants claimed by some of the original colonies. In time, these grants were ceded to the federal government.
The first great expansion of the country came with the Louisiana Purchase of 1803, which doubled the country's territory, although the southeastern border with Spanish Florida was the subject of much dispute until it and Spanish claims to the Oregon Country were ceded to the US in 1821. The Oregon Country gave the United States access to the Pacific Ocean, though it was shared for a time with the United Kingdom. The annexation of the Republic of Texas in 1845 led directly to the Mexican–American War, after which the victorious United States obtained the northern half of Mexico's territory, including what became the state of California.
As the country expanded westward, the question of slavery became increasingly prominent, with debate over whether new territories would permit it, and events such as the Missouri Compromise and Bleeding Kansas reflecting that tension. Southern state governments proclaimed secession in 1860 and 1861, forming the Confederate States of America. The American Civil War ended in the defeat of the Confederacy in 1865 and the eventual readmission of the southern states to the Congress.
The United States began expanding beyond North America in 1856 with the passage of the Guano Islands Act, causing many small and uninhabited, but economically important, islands in the Caribbean Sea and the Pacific Ocean to be claimed. Most of these claims were eventually abandoned, largely because of competing claims from other countries. The Pacific expansion culminated in the annexation of Hawaii in 1898, after the overthrow of its government five years previously. Alaska, the last major acquisition in North America, was purchased from Russia in 1867. Support for the independence of Cuba from the Spanish Empire, and the sinking of the USS Maine, led to the Spanish–American War in 1898, in which the United States gained Puerto Rico, Guam, and the Philippines, and occupied Cuba for several years. American Samoa was acquired in 1900 after the end of the Second Samoan Civil War. The United States purchased the U.S. Virgin Islands from Denmark in 1917.
Following the war, many islands were entrusted to the U.S. by the United Nations, and while the Northern Mariana Islands became a U.S. territory, the Marshall Islands, Federated States of Micronesia, and Palau emerged from the trust territory as independent nations. The last major international change was the acquisition in 1904, and return to Panama in 1979, of the Panama Canal Zone, an unincorporated US territory which controlled the Panama Canal. The final transfer of formal control over the region to Panama was completed in 1999.
States have generally retained their initial borders once established. Only three states (Kentucky, Maine, and West Virginia) have been created directly from area belonging to another state (although at the time of admission, Vermont agreed to a monetary payment for New York to relinquish its claim); all of the other states were created from federal territories or from acquisitions. Four states (Louisiana, Missouri, Nevada, and Pennsylvania) have expanded substantially by acquiring additional federal territory after their initial admission to the Union. In 1912, Arizona was the last state established in the contiguous United States. In 1959, Hawaii became the 50th and most recent state admitted.
1776–1784 (American Revolution)
DateEventChange mapJuly 4, 1776Thirteen colonies of the Kingdom of Great Britain in North America collectively declared their independence as the United States of America, though several colonies had already individually declared independence: The Colony of Connecticut, becoming the State of Connecticut; The Counties of New Castle, Kent, and Sussex, upon Delaware, commonly known as the Lower Counties on Delaware, which was Delaware Colony before their declaration of independence on June 15, 1776; The Province of Georgia, becoming the State of Georgia; The Province of Maryland, becoming the State of Maryland; The Province of Massachusetts Bay, becoming the State of Massachusetts Bay; The State of New Hampshire, which was the Province of New Hampshire before their declaration of independence on June 15, 1776; The Province of New Jersey, becoming the State of New Jersey; The Province of New York, becoming the State of New York; The Province of North Carolina, becoming the State of North Carolina; The Province of Pennsylvania, becoming the State of Pennsylvania; The State of Rhode Island and Providence Plantations, commonly known as Rhode Island, which was the Colony of Rhode Island and Providence Plantations before declaring independence on May 4, 1776; The Province of South Carolina, becoming the State of South Carolina; The Colony of Virginia, becoming the Commonwealth of Virginia.
The capital was not specifically established; at the time, the Continental Congress met in Philadelphia.
Many states had only vaguely defined borders at the time of independence, with only the far eastern borders having been fully mapped. The borders of North Carolina were particularly poorly surveyed, its border with South Carolina having been done in several pieces, none of which truly matched the spirit of the charter, and its border with Virginia veered several miles north of ideal.
Virginia claimed the vast northwest across the Ohio River, and Connecticut, Massachusetts Bay, and New York also claimed areas west of their settled borders, overlapping with Virginia's claim.
Some colonies had historical "sea to sea" grants, but British claims west of the Mississippi were renounced in favor of the claims of France to Louisiana in the 1763 Treaty of Paris. The Royal Proclamation of 1763 limited the Thirteen Colonies to settlement east of the Appalachian Mountains, but the colonies that became states continued to claim territory west of this line. The Quebec Act of 1774 granted territory between the Great Lakes, Mississippi River, and the Ohio River (which partly overlapped these continued claims) to the newly British colony of Quebec, a grievance mentioned in the Declaration of Independence and considered one of the Intolerable Acts.
The entirety of the new United States was claimed by Great Britain, including Machias Seal Island and North Rock, two small islands off the northeast coast which remain disputed up to the present.September 21, 1776The Counties of New Castle, Kent, and Sussex, upon Delaware enacted a constitution, becoming the Delaware State.September 28, 1776The State of Pennsylvania enacted a constitution, becoming the Commonwealth of Pennsylvania.December 20, 1776To avoid British forces who were advancing on Philadelphia, the Continental Congress began meeting in Baltimore.January 17, 1777The northeastern region of New York, known as the New Hampshire Grants, declared independence as New Connecticut.March 4, 1777The Continental Congress returned to Philadelphia after the threat to it by British forces ended.June 4, 1777New Connecticut was renamed Vermont.Vermont 2006 J.R.H. 3, Joint Resolution Designating January as Vermont History and Independence Month, accessed October 19, 2025. September 27, 1777The Continental Congress fled Philadelphia after the American defeat at the Battle of Brandywine, and briefly met in Lancaster, Pennsylvania.September 30, 1777The Continental Congress continued to move away from Philadelphia, settling in York, Pennsylvania.June 11, 1778Vermont claimed what was called the "East Union", consisting of some towns in New Hampshire that petitioned to join with Vermont, out of concern that their state was focusing too much on its coastal region.July 2, 1778The Continental Congress returned to Philadelphia following British withdrawal.October 21, 1778Under pressure from the Continental Congress, Vermont rescinded the annexation of the East Union; the legislature declared on February 12, 1779, that the East Union should be considered null from its beginning.October 25, 1780The State of Massachusetts Bay enacted a constitution, becoming the Commonwealth of Massachusetts.March 1, 1781The Articles of Confederation entered into force. The British Province of Quebec was invited to join, but did not do so.no change to mapApril 5, 1781Vermont again claimed an East Union, consisting of some towns in New Hampshire that wished to join with Vermont, more than during the first attempt in 1778.June 16, 1781Vermont claimed what was called the "West Union", consisting of some towns in New York, mainly to counterbalance Vermont's attempt at eastward expansion.February 23, 1782Vermont abandoned its attempts to annex the East Union from New Hampshire and the West Union from New York.October 29, 1782The western claim of New York was ceded to the federal government. New York proclaimed its new western border to be a line drawn south from the western end of Lake Ontario. At its maximum interpretation, the state ceded a claim bounded by Lake Erie, Lake Huron and Lake Michigan on the north; the Illinois, Mississippi, and Tennessee Rivers on the west and south; and the Appalachian Mountains and Pennsylvania on the east. The cession included the tip of New York north of Pennsylvania, which would be known as the Erie Triangle.December 30, 1782The Congress of the Confederation declared that the region that Connecticut claimed as Westmoreland County was part of Pennsylvania, attempting to end the Pennamite–Yankee War.June 30, 1783The Pennsylvania Mutiny, and the state's refusal to stop it, caused the Congress of the Confederation to leave Philadelphia for Princeton.November 26, 1783The Congress of the Confederation reconvened in Annapolis.March 1, 1784The claim of Virginia northwest of the Ohio River was ceded to the federal government.April 1, 1784Pennsylvania and Virginia agreed to set their border as continuing the Mason–Dixon line to a point five degrees west of the Delaware River, then north.May 12, 1784Great Britain recognized the independence of the United States, ending its claim to the country. The treaty ended the American Revolutionary War, though military action had largely ended after the Franco-American victory at Yorktown on October 19, 1781.
Because of ambiguities and poor knowledge of geography, the treaty was unclear in several areas: The northeastern border was defined as running from the headlands of the St. Croix River, but there were several options for that river. The northwestern border was defined as a line running west from the northwestern point of the Lake of the Woods to the Mississippi River, but the river's source was south of that The border extending from Lake Superior towards the Lake of the Woods was poorly defined as starting from "Long Lake", which both sides interpreted to their advantage. Machias Seal Island and North Rock off the northeast coast, and Sugar Island off the Upper Peninsula of Michigan, remained disputed.