Geography Timeline: 13 Key Moments That Changed U.S. Boundaries

History of U.S. Expansion and Boundary Changes Since 1776

Map of the United States. Getty Images

The United States of America was founded in 1776 along the east coast of North America, wedged between British Canada and Spanish Mexico. The original country consisted of thirteen states and territory that extended west to the Mississippi River. Since 1776, a variety of treaties, purchases, wars, and Acts of Congress have extended the territory of the United States to what we know today.

The U.S. Senate (the upper house of Congress) approves treaties between the United States and other countries. However, boundary changes of states that lie on international borders require the approval of the state legislature in that state. Boundary changes between states require the approval of each state's legislature and the approval of Congress. The U.S. Supreme Court settles boundary disputes between states.

The 18th Century

Between 1782 and 1783, treaties with the United Kingdom establish the U.S. as an independent country and establish the boundary of the United States as being bound on the north by Canada, on the south by Spanish Florida, on the west by the Mississippi River, and on the east by the Atlantic Ocean.

The 19th Century

The 19th century was the most important period in the United States' expansion, thanks in part to the widespread acceptance of the idea of manifest destiny, that it was America's special, god-given mission to expand westward. 

This expansion started out with the hugely-consequential Louisiana Purchase in 1803, which extended the western boundary of the United States to the Rocky Mountains, occupying the drainage area of the Mississippi River. The Louisiana Purchase doubled the territory of the United States.

In 1818, a convention with the United Kingdom expanded this new territory even further, establishing the northern boundary of the Louisiana Purchase at 49 degrees north.

Just a year later, in 1819, Florida was ceded to the United States and purchased from Spain.

At the same time, the United States was expanding northward. In 1820, Maine became a state, carved out of the state of Massachusetts. The northern boundary of Maine was disputed between the U.S. and Canada so the King of the Netherlands was brought in as an arbiter and he settled the dispute in 1829. However, Maine refused the deal and since Congress requires the approval of a state legislature for boundary changes, the Senate could not approve a treaty over the border. Ultimately, in 1842 a treaty established the Maine-Canada border of today although it provided Maine with less territory than the King's plan would have.

The independent Republic of Texas was annexed to the United States in 1845. The territory of Texas extended north to 42 degrees north (into modern Wyoming) due to a secret treaty between Mexico and Texas.

In 1846, Oregon Territory was ceded to the U.S. from Britain following an 1818 joint claim on the territory, which resulted in the phrase "Fifty-Four Forty or Fight!". The Treaty of Oregon established the boundary at 49 degrees north.

Following the Mexican War between the U.S. and Mexico, the countries signed the 1848 Treaty of Guadalupe, resulting in the purchase of Arizona, California, Nevada, New Mexico, Texas, Utah, and western Colorado.

With the Gadsden Purchase of 1853, the land acquisition that resulted in the area of the 48 contiguous states today was completed. Southern Arizona and southern New Mexico were purchased for $10 million and named for the U.S. minister to Mexico, James Gadsden.

When Virginia decided to secede from the Union at the start of the Civil War (1861-1865), the western counties of Virginia voted against the secession and decided to form their own state. West Virginia was established with help from Congress, who approved of the new state on December 31, 1862 and West Virginia was admitted to the Union on June 19, 1863. West Virginia was originally going to be called Kanawha.

In 1867, Alaska was purchased from Russia for $7.2 million in gold. Some thought the idea was ridiculous and the purchase became known as Seward's Folly, after Secretary of State William Henry Seward. The boundary between Russia and Canada was established by treaty in 1825.

In 1898,  Hawaii was annexed into the United States.

The 20th Century

In 1925, a final treaty with the United Kingdom clarified the boundary through the Lake of the Woods (Minnesota), resulting in the transfer of a few acres between the two countries.

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Rosenberg, Matt. "Geography Timeline: 13 Key Moments That Changed U.S. Boundaries." ThoughtCo, Apr. 5, 2023, Rosenberg, Matt. (2023, April 5). Geography Timeline: 13 Key Moments That Changed U.S. Boundaries. Retrieved from Rosenberg, Matt. "Geography Timeline: 13 Key Moments That Changed U.S. Boundaries." ThoughtCo. (accessed May 29, 2023).