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 The Second Continental Congress was a late-18th-century meeting of delegates from the Thirteen Colonies that united in support of the American Revolutionary War. The Congress was creating a new country it first named "United Colonies" and in 1 ...

two days prior, the colonies resolved that they were free and independent states. The union was formalized in the

Articles of Confederation The Articles of Confederation and Perpetual Union was an agreement among the 13 Colonies of the United States of America that served as its first frame of government. It was approved after much debate (between July 1776 and November 1777) by ...

, 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 effectively doubled the size of the colonies, now able to stretch west past the

Proclamation Line The Royal Proclamation of 1763 was issued by King George III on 7 October 1763. It followed the Treaty of Paris (1763), which formally ended the Seven Years' War and transferred French territory in North America to Great Britain. The Proclam ...

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 The act of cession is the assignment of property to another entity. In international law it commonly refers to land transferred by treaty. Ballentine's Law Dictionary defines cession as "a surrender; a giving up; a relinquishment of jurisdictio ...

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 Spanish Florida ( es, La Florida) was the first major European land claim and attempted settlement in North America during the European Age of Discovery. ''La Florida'' formed part of the Captaincy General of Cuba, the Viceroyalty of New Spain, ...

was the subject of much dispute until it and Spanish claims to the

Oregon Country Oregon Country was a large region of the Pacific Northwest of North America that was subject to a long dispute between the United Kingdom and the United States in the early 19th century. The area, which had been created by the Treaty of 1818, co ...

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 The Republic of Texas ( es, República de Tejas) was a sovereign state in North America that existed from March 2, 1836, to February 19, 1846, that bordered Mexico, the Republic of the Rio Grande in 1840 (another breakaway republic from Mex ...

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 was quickly made the state of California. However, as the development of the country moved west, the question of slavery became more important, with vigorous debate over whether the new territories would allow slavery and events such as the Missouri Compromise and Bleeding Kansas. This came to a head in 1860 and 1861, when the governments of the

southern states Southern States may refer to: *The independent states of the Southern hemisphere United States * Southern United States, or the American South * Southern States Cooperative, an American farmer-owned agricultural supply cooperative * Southern Stat ...

proclaimed their secession from the country and formed the Confederate States of America. The American Civil War led to the defeat of the Confederacy in 1865 and the eventual readmission of the states to the United States Congress. The cultural endeavor and pursuit of

manifest destiny Manifest destiny was a cultural belief in the 19th century in the United States, 19th-century United States that American settlers were destined to expand across North America. There were three basic tenets to the concept: * The special vir ...

provided a strong impetus for westward expansion in the 19th century. 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 due to competing claims from other countries. The Pacific expansion culminated in the

annexation of Hawaii Hawaii ( ; haw, Hawaii or ) is a U.S. state, state in the Western United States, Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is ...

in 1898, after the

overthrow Overthrow may refer to: * Overthrow, a change in government, often achieved by force or through a coup d'état. **The 5th October Overthrow, or Bulldozer Revolution, the events of 2000 that led to the downfall of Slobodan Milošević in the former ...

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 The Cuban War of Independence (), fought from 1895 to 1898, was the last of three liberation wars that Cuba fought against Spain, the other two being the Ten Years' War (1868–1878) and the Little War (1879–1880). The final three months ...

from the Spanish Empire, and the sinking of the

USS Maine Four ships of the United States Navy have borne the name USS ''Maine'', named for the 23rd state: * , was a battleship whose 1898 sinking precipitated the Spanish–American War. * , launched in 1901, was the lead ship of her class of battleships ...

, 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 by the United States in 1900 after the end of the Second Samoan Civil War. The United States purchased the U.S. Virgin Islands from Denmark in 1917. Puerto Rico and Guam remain territories, and the Philippines became independent in 1946, after being a major theater of World War II. 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 The Panama Canal Zone ( es, Zona del Canal de Panamá), also simply known as the Canal Zone, was an unincorporated territory of the United States, located in the Isthmus of Panama, that existed from 1903 to 1979. It was located within the terr ...

, an unincorporated US territory which controlled the Panama Canal. The final cession of formal control over the region was made to Panama 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 The contiguous United States (officially the conterminous United States) consists of the 48 adjoining U.S. states and the Federal District of the United States of America. The term excludes the only two non-contiguous states, Alaska and Hawaii ...

, commonly called the "lower 48". In 1959, Hawaii was the 50th and most recent state admitted.

Legend for maps

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1776–1784 (American Revolution)

1784–1803 (Organization of territory)

1803–1818 (Purchase of Louisiana)

1819–1845 (Northwest expansion)

1845–1860 (Southwest expansion)

1860–1865 (Civil War)

1866–1897 (Reconstruction and western statehood)

1898–1945 (Pacific and Caribbean expansion)

1946–present (Decolonization)

Bancos along the Rio Grande

The

Banco Convention of 1905 The Banco Convention of 1905, between the United States and Mexico, was negotiated and signed in 1905 and ratified by both nations in 1907. It resulted in 247 exchanges of bancos (land surrounded by bends in the river that became segregated by a ...

between the United States and Mexico allowed, in the event of sudden changes in the course of the

Rio Grande The Rio Grande ( and ), known in Mexico as the Río Bravo del Norte or simply the Río Bravo, is one of the principal rivers (along with the Colorado River) in the southwestern United States and in northern Mexico. The length of the Rio G ...

(as by flooding), for the border to be altered to follow the new course. The sudden changes often created ''bancos'' (land surrounded by bends in the river that became segregated from either country by a cutoff, often due to rapid accretion or avulsion of the alluvial channel), especially in the Lower Rio Grande Valley. When these bancos are created, the International Boundary and Water Commission investigates if land previously belonging to the United States or Mexico is to be considered on the other side of the border. In all cases of these adjustments along the Rio Grande under the 1905 convention, which occurred on 37 different dates from 1910 to 1976, the transferred land was minuscule (ranging from one to 646 acres) and uninhabited.