When Mexico won independence from Spain in 1821, it inherited a vast territory stretching from the Pacific Coast deep into the North American interior. Over the next quarter-century, those lands—known collectively as Alta California, Nuevo Mexico, and parts of Texas—remained under Mexican sovereignty and were populated with missions and early settlements. But the discovery of gold in California, American expansionist ambitions fueled by Manifest Destiny, and disputes over Texas’s border led to war between the neighboring countries. In February 1848, the Treaty of Guadalupe Hidalgo ended the conflict and permanently reshaped the continent’s map, transferring vast territories from Mexico to the U.S.
Today, these former Mexican lands are facing a deepening migratory crisis, which escalated over the past weekend as the Trump administration deployed both the National Guard and Marines to the city of Los Angeles in response to riots against ICE.
The situation has also put President Trump at odds with California Governor Gavin Newsom, marking the first time a president has overruled a state governor to deploy the National Guard since 1965.
The Treaty of Guadalupe Hidalgo
On February 2, 1848, in the town of Guadalupe Hidalgo just outside Mexico City, delegates from both nations signed the “Treaty of Peace, Friendship, Limits, and Settlement,” ending the Mexican–American War that had lasted from April 1846 to February 1848. The conflict was triggered by the United States’ 1845 annexation of Texas, which Mexico still claimed. The dispute centered on whether Texas’s southern border lay at the Nueces River or the Rio Grande. Historians also cite the expansionist doctrine of Manifest Destiny, which drove U.S. leaders to pursue continental expansion, as a major cause of the war.
Under President James K. Polk’s direction, U.S. forces marched into disputed territory on the Rio Grande, provoking engagements at Palo Alto and Resaca de la Palma in May 1846, and later won crucial battles at Buena Vista and Vera Cruz. In March 1847, General Winfield Scott landed troops at Veracruz and then moved inland. US forces captured Mexico City in September 1847.
Mexico eventually agreed to negotiate. The resulting treaty formalized Mexico’s defeat and concession of roughly 525,000 square miles, which was about 55 percent of its territory, to the U.S. in exchange for $15 million and the assumption of American claims against Mexico.
Mexico relinquished all claims to Texas, recognized the Rio Grande as the new southern boundary, and secured guarantees for the property and civil rights of Mexicans in the ceded lands. The U.S. Senate ratified the treaty on March 10, 1848, and ratifications were exchanged in May, extending the U.S.’ western border to the Pacific and laying the groundwork for ten future states.
From Mexico to US States
After the treaty was signed, American settlers came into the former Mexican territories. They were mainly drawn by gold, silver, fertile valleys, and religious freedom. Over the next six decades, ten states emerged from that territory cession.
California
Immediately after the treaty was signed, rumors of gold in California caused settlers to storm into those lands. California had previously been a remote, ranch-and-mission land under Mexican rule, and was quickly transformed into the nation’s most attractive locations in the United States. California gained statehood in September 1850, its population and economy soaring on the promise of “ the forty-niners’ gold rush” and new ports.
Nevada
The U.S. state now known as Nevada was actually called Alta California under Mexican rule. After silver was discovered in the Comstock Lode in 1859, “The Silver State” attracted thousands, and Congress took Nevada out of the western Utah Territory. By October 31, 1864, in the middle of the American Civil War, Nevada’s gain of statehood boosted the Union with its mineral wealth, mainly to fund the war effort.
Arizona
Arizona became an American state in 1848, but it remained administratively tied to New Mexico Territory until Congress created Arizona Territory in 1863. Despite its hostile geography, the state’s canyons and plateaus soon drew ranchers, miners, and railroads. On Valentine’s Day 1912, Arizona finally gained statehood, completing becoming the fourth Southwest state born of the treaty.
Utah
In 1847, Brigham Young led Mormon pioneers into the Great Salt Lake Valley. At the time, it was still under Mexican rule. Young hoped to establish “Deseret,” a proposed US State promoted by the followers of the Church of Jesus Christ. A year later, the Treaty of Guadalupe Hidalgo placed the valley in U.S. hands. Congress organized Utah Territory in 1850, but it took nearly half a century, and its inhabitants’ renunciation of polygamy, before Utah joined the Union on January 4, 1896
Texas
Texas had already seceded from Mexico in 1836 and existed as an independent republic before its 1845 annexation by the United States. Mexico, however, never recognized that move until the 1848 treaty, which finally confirmed the Rio Grande as Texas’s southern border and settled lingering disputes over the State
Colorado, New Mexico, Wyoming, Kansas, Oklahoma
While California, Nevada, Arizona, and Utah drew most attention, the treaty also transferred parts of what would become five other states. Colorado’s eastern plains and mountains, named for the “reddish river” the Spaniards called Río Colorado, fell into U.S. custody and became a territory in 1861 before finally becoming a state in 1876.
New Mexico, with its high desert and Hispanic settlements, was organized as a territory in 1850 and gained statehood in 1912. Wyoming inherited a sliver of Mexico’s northwestern reach. It joined the Union in 1890. Southwestern Kansas and the slender Panhandle of Oklahoma, once under Mexican jurisdiction, rounded out the list, becoming parts of their respective territories in the 1850s and 1890s and states by 1907.