The United States was formed in two overlapping processes: a political founding between 1776 and 1789, and a territorial expansion that ran from 1783 to 1959. Thirteen British colonies on the Atlantic coast declared independence in Philadelphia on 4 July 1776, won recognition in the Treaty of Paris of 1783, and adopted the Constitution that took effect on 4 March 1789. The country then grew westward by purchase, treaty and war: the Louisiana Purchase from France in 1803, Florida from Spain in 1819, Texas in 1845, the Oregon Country in 1846, the Mexican Cession in 1848, the Gadsden Purchase in 1853–54 and Alaska from Russia in 1867. Arizona became the 48th and last contiguous state on 14 February 1912, and Alaska and Hawaii brought the total to 50 in 1959. Almost all of that land was already home to Native American nations, whose removal and dispossession is part of the same story.
Lexington, Independence Hall and Yorktown: Winning the Thirteen States
Fighting between British troops and colonial militias began at Lexington and Concord in Massachusetts on 19 April 1775. The Second Continental Congress, meeting in the Pennsylvania State House in Philadelphia, the building now called Independence Hall, adopted the Declaration of Independence on 4 July 1776. The thirteen colonies that signed it stretched from New Hampshire to Georgia, hemmed in to the west by the Appalachian Mountains and by a British proclamation that had closed the lands beyond them to settlement.
The war turned at Yorktown in Virginia, where a combined American and French force under George Washington and the Comte de Rochambeau, with a French fleet under the Comte de Grasse blocking Chesapeake Bay, forced the surrender of Lord Cornwallis’s army on 19 October 1781. Peace talks opened in Paris in April 1782. The Treaty of Paris, signed on 3 September 1783, recognised the thirteen states as free and independent and gave them a far larger territory than they had held as colonies: all the land east of the Mississippi River, north of Spanish Florida and south of British Canada. British negotiators themselves described these boundaries as exceedingly generous. British forts stayed in the Great Lakes country for years and the Native nations of the Ohio Valley had not been party to the treaty at all.
How the Northwest Ordinance Set the Rule for New States
The first national government was weak by design. The Articles of Confederation came into force on 1 March 1781, once all thirteen states had ratified them, and gave Congress no power to tax or to regulate trade. Yet that Congress made one decision that shaped the whole later map. The Northwest Ordinance of 13 July 1787 organised the Northwest Territory, the land north-west of the Ohio River and east of the Mississippi, which would later form Ohio, Indiana, Illinois, Michigan, Wisconsin and part of Minnesota. It established that territories would become new states equal to the original thirteen rather than colonies of them, and it prohibited slavery in the territory. That principle of admitting states on equal terms is how the country grew from 13 to 50.
The same summer, delegates met in the State House in Philadelphia and drafted a new Constitution between 25 May and 17 September 1787. State conventions ratified it through 1787 and 1788, and it replaced the Articles on 4 March 1789. George Washington took the oath as the first president on 30 April 1789 on the balcony of Federal Hall in New York City, then the capital. The first census, taken on 2 August 1790, counted 3,929,214 people. That July, the Residence Act chose a permanent seat of government on the Potomac River, made Philadelphia the temporary capital and set a deadline of December 1800 for the new city of Washington to be ready.
Jefferson’s Louisiana, Spain’s Florida and the 36°30′ Line
The biggest single change to the map came in 1803. Napoleon had recovered Louisiana from Spain in 1800, but after France failed to suppress the revolt in Saint-Domingue, today’s Haiti, and faced renewed war with Britain, he sold it. For $15 million, roughly $18 per square mile, the United States acquired about 2,140,000 km² (828,000 sq mi), most of the Mississippi’s drainage basin west of the river, from New Orleans to the northern plains. France controlled only a small part of this area. For most of it, the United States had bought the right, against other colonial powers, to take land from its Native American inhabitants by treaty or by conquest. The deal is explored in more detail in our Louisiana Purchase guide.
Florida followed. In the Adams–Onís Treaty, signed on 22 February 1819 by Secretary of State John Quincy Adams and the Spanish minister Luis de Onís, Spain ceded Florida, the United States gave up its claim to Texas, and the two countries drew the first boundary running all the way to the Pacific. Expansion immediately raised the question of slavery. The Missouri Compromise, signed by President James Monroe on 6 March 1820, admitted Missouri as a slave state and Maine as a free state, and banned slavery in the rest of the Louisiana lands north of latitude 36°30′. Arguments over which new territories would allow slavery ran through every later acquisition until the Civil War.
The Trail of Tears, the 49th Parallel and the Rio Grande
Expansion on paper needed land in practice, and the government took it from Native nations. The Indian Removal Act, signed by President Andrew Jackson on 28 May 1830, provided for exchanging land with tribes in the existing states for land west of the Mississippi. Under Jackson and Martin Van Buren, about 60,000 members of the Cherokee, Muscogee (Creek), Seminole, Chickasaw and Choctaw nations, including thousands of people they held as slaves, were moved from the south-east to the Indian Territory in present-day Oklahoma, and thousands died of exposure, disease and starvation on the journeys known as the Trail of Tears. The Cherokee removal of 1838 followed the discovery of gold near Dahlonega, Georgia.
The south-west came next. Texas declared independence from Mexico on 2 March 1836 and was admitted as the 28th state on 29 December 1845. In the north-west, Britain and the United States had jointly occupied the Oregon Country since 1818; the Oregon Treaty of 15 June 1846 split it along the 49th parallel, the line that still divides Washington State from British Columbia. War with Mexico broke out over the Texas border in 1846. After American troops took Mexico City, the Treaty of Guadalupe Hidalgo of 2 February 1848 forced Mexico to cede land which, together with Texas, amounted to about 55 per cent of its pre-war territory. The Mexican Cession alone, excluding Texas, covered roughly 1,370,000 km² (529,000 sq mi) and took in today’s California, Nevada and Utah and most or parts of Arizona, New Mexico, Colorado and Wyoming. The United States paid $15 million. The Gadsden Purchase, which took effect on 8 June 1854, added 76,800 km² (29,640 sq mi) south of the Gila River for $10 million, wanted as a flat route for a southern transcontinental railway, and fixed the present Mexico–United States border.
Fort Sumter to Honolulu: Secession, Alaska and the Last Two Stars
The quarrel over slavery in the new territories finally split the country. Abraham Lincoln, who opposed letting slavery spread west, won the 1860 election. Seven southern states (South Carolina, Mississippi, Florida, Alabama, Georgia, Louisiana and Texas) then seceded and formed the Confederate States of America on 8 February 1861. Virginia, Arkansas, Tennessee and North Carolina joined them after Confederate forces fired on Fort Sumter in April 1861. The Union’s victory in 1865 settled that no state could leave, and the war redrew one boundary permanently: the western counties of Virginia, which rejected secession, were admitted as the new state of West Virginia on 20 June 1863.
After the Civil War the map grew beyond the continent. Russia, defeated in the Crimean War and unable to defend its distant colony, sold Russian America to the United States. Secretary of State William Seward and the Russian diplomat Eduard de Stoeckl signed the treaty on 30 March 1867, the Senate ratified it on 9 April, and American sovereignty took effect on 18 October. For $7.2 million, about two cents an acre, the country grew by 1,518,800 km² (586,412 sq mi), its second-largest acquisition after Louisiana.
Meanwhile the continental territories filled with settlers and were admitted one by one. New Mexico became the 47th state on 6 January 1912 and Arizona the 48th on 14 February 1912, completing the contiguous 48. Hawaii took a different road. Its queen, Liliʻuokalani, was overthrown in a coup in Honolulu on 17 January 1893, led by a Committee of Safety and backed by US Marines and sailors sent ashore by the American minister, John L. Stevens. The islands were annexed by the Newlands Resolution of 7 July 1898 and organised as a territory in 1900. Alaska became the 49th state on 3 January 1959 under the Alaska Statehood Act, and Hawaii the 50th on 21 August 1959, the most recent admission to the Union. The territories that never became states, such as Puerto Rico and Guam, remain outside the fifty. For a full breakdown of the present-day divisions, see our guide to how many states are in the United States, and for the country’s total area today, how big the United States is.