Except where otherwise attributed, maps used under the Creative Commons Attribution-Share Alike 3.0 Unported license, courtesy of "Golbez". This article is adapted from multiple Wikipedia articles, including Territorial Evolution of the United States.
NOTE: Territorial changes in areas outside the southwestern United States are ignored for purposes of this article.
1. The territorial growth of the southwestern United States begins in 1819, with the signing of the Adams-Onis Treaty. That treaty, besides including the terms under which the US purchased Spanish Florida, defined the borders between the Viceroyalty of New Spain (later Mexico) and the United States. Pre-treaty borders were highly irregular and the treaty simplified the borders. The new US-Mexico boundary was to be the Sabine River (western border of Louisiana) and north from the Gulf of Mexico to the 32nd parallel north, then due north to the Red River, west along the Red River to the 100th meridian west, due north to the Arkansas River, west to its headwaters, north to the 42nd parallel north, and finally west along that parallel to the Pacific Ocean.
2. On September 27, 1821, the Viceroyalty of New Spain declared independence as Mexico.
3, On March 2, 1836, the Republic of Texas declared its independence from Mexico. Texas' boundaries with Mexico were disputed at the time of Texas' claim of independence. As shown on the map below, the actual territory of the Republic of Texas was much smaller than the disputed territories between the Republic of Texas and Mexico.
4. On December 29, 1845, the Republic of Texas was admitted to the US as the state of Texas by the Texas Annexation. The boundaries of Texas, though not set out in the Joint Annexation Resolution, included much of the territory that had been disputed between the Republic of Texas and Mexico.
5. From 1846-48, the Mexican-American War was fought. The Treaty of Guadalupe Hidalgo (1848), which marked the end of the war, redefined the US/Mexico border. From east to west, the new border consisted of the Rio Grande northwest from its mouth to the point Santa Fe de Nuevo Mexico (roughly 32 degrees north), then due west from this point to the 110th meridian west, then north along the 110th Meridian to the Gila River and down the river to its mouth. In the US, the area north of this new national border was known as the Mexican Cession, and was unorganized US territory.
6. In 1850, the California Republic was admitted to the US under the Compromise of 1850 as the state of California. The changes in governance of Alta California to the California Republic to the State of California is worthy of a separate post to come soon.
Additionally, the unorganized Mexican Cession territory was divided into the Utah and New Mexico Territories. Much of what had been claimed as western Texas was given to the New Mexico Territory by Texas in exchange for forgiveness of Texas debt by the US government.
7. In 1854, The Gadsden Purchase was signed by President Franklin Pierce. The Purchase gave the US all Mexican territory south of the Gila River and west of the Rio Grande. This strip was purchased for its perceived value in the US for constructing a transcontinental railroad.
At this point, all territory that would become the lower 48 contiguous states was acquired by the government of the United States.
9. On March 2, 1961, Utah Territory was divided into Utah and Nevada Territories. The Nevada Territory was smaller in area than the current state of Nevada and the Utah Territory was larger in area than the current state of Utah.
10. 1861- ; The US Civil War created conflicting territorial claims to/on the New Mexico Territory.
The Confederate States of America claimed to split New Mexico Territory into New Mexico Territory (North) and Arizona Territory (South).
On the other hand, the USA, split New Mexico Territory into New Mexico Territory (east) and Arizona Territory (west). Furthermore, the eastern border the Nevada Territory was moved further east, increasing the size of the Nevada Territory at the expense of the size of the Utah Territory.
11. On October 31, 1864, Nevada Territory was admitted to the US as the state of Nevada. At the time of admission, Nevada did not have its distinctive south-eastern triangular portion, which would become Clark County and give Nevada access to the Colorado River.
12. 1865-66: With the end of the Civil War, Confederate States of America claims on Arizona and New Mexico Territories were dropped. The state of Nevada was also given more Utah Territory land (eastern Nevada-Utah Territory border moved further east).
13. On January 18, 1867, Nevada was given its south-eastern triangular portion, giving Nevada access to the Colorado River. This land was from territory taken away from Arizona Territory. This territorial swap finalized Nevada's current borders.
14. On August 1, 1876, Colorado Territory was admitted to the US as the state of Colorado.
15. On January 4, 1896, Utah Territory was admitted to the US as the state of Utah.
16. On January 6, 1812, New Mexico Territory was admitted to the US as the state of New Mexico. In 1907, Oklahoma Territory and Indian Lands were combined and entered into the Union as the state of Oklahoma.
17. On February 14, 1912, Arizona Territory was admitted to the US as the state of Arizona; the last of the contiguous states to be admitted.
Order and years of Admission for the Southwestern States of the US
Texas (28th) (1841)
California (31st) (1850)
Nevada (36th) (1864)
Colorado (38th) (1876)
Utah (45th) (1896)
Oklahoma (46th) (1907)
New Mexico (47th) (1912)
Arizona (48th) (1912)