Mexican Cession

The Mexican Cession (Spanish: Cesión mexicana) is the region in the modern-day Western United States that Mexico previously controlled, then ceded to the United States in the Treaty of Guadalupe Hidalgo in 1848 after the Mexican–American War. This region had not been part of the areas east of the Rio Grande that had been claimed by the Republic of Texas, which had been claiming independence since its Texas Revolution of 1836 and subsequent brief war for independence, followed afterwards a decade later by the American annexation and admitted statehood in 1845. It had not specified the southern and western boundary of the new state of Texas with New Mexico. Consisting of roughly 529,000 square miles (1,370,000 km²), not including any Texas lands, the Mexican Cession was the third-largest acquisition of territory in US history, surpassed only by the 827,000-square-mile (2,140,000 km²) Louisiana Purchase of 1803 and the later 586,000-square-mile (1,520,000 km²) Alaska Purchase from Russia in 1867.

Most of the area had been considered the Mexican territory and province of

The northern boundary of the territories ceded by Mexican to the US was at the 42nd parallel north of latitude was originally set by the Adams–Onís Treaty of arbitration and border settlement signed two decades before by the United States and the Kingdom of Spain in 1821 after a long dispute and was further ratified by the successor state of an independent republic in Mexico in 1831 in the Treaty of Limits between them then. The eastern boundary of the Mexican Cession was the former old Texas Republic claim of additional western lands from the time of their Revolution of 1836 set at the Rio Grande and extending north to the headwaters of the Rio Grande, not corresponding to Mexican territorial boundaries. The southern boundary was set by the war-ending peace Treaty of Guadalupe Hidalgo, which used and followed the original Mexican boundaries between Alta California (to the north) and Baja California (Lower California) and the Mexican state of Sonora (to the south).

Until the American Civil War (1861–1865), the question of whether future Western states formed out of the 1848 Mexican Cession lands would or would not permit the institution of slavery in the newly acquired territories was a major American political issue, and was one of the causes of the eventual conflict.

Alta California and Santa Fe de Nuevo México were captured soon after the start of the war and the last resistance there was subdued in January 1847, but Mexico would not accept the loss of territory. Therefore, during 1847, troops from the United States invaded central Mexico and occupied the Mexican capital of Mexico City, but still no Mexican government was willing to ratify the transfer of the northern territories to the US. It was uncertain whether any treaty could be reached. There was even an All of Mexico Movement proposing complete annexation of Mexico among Eastern Democrats but opposed by Southerners like John C. Calhoun who wanted the additional territory for their crops but not the large population of central Mexico.^{[citation needed]}

Eventually Nicholas Trist forged the Treaty of Guadalupe Hidalgo, explicitly redefining the border between Mexico and the United States in early 1848 after President Polk had already attempted to recall him from Mexico as a failure. Although Mexico did not overtly cede any land under the treaty, the redefined border had the effect of transferring Alta California and Santa Fe de Nuevo México to the control of the United States. Equally important, the new border also acknowledged Mexico's loss of Texas, both the core eastern portion and the western claims, neither of which had been formally recognized by Mexico until that time.^{[citation needed]}

The US Senate approved the treaty, rejecting amendments from Jefferson Davis to also annex most of northeastern Mexico and from Daniel Webster to decline to take Alta California and Santa Fe de Nuevo México.^[1] The United States paid $15 million ($482 million in 2016 dollars) for the damage caused by the war in Mexico's territory and agreed to assume $3.25 million in debts to US citizens.^[2]

The Mexican Cession as ordinarily understood (i.e. excluding lands claimed by Texas) amounted to 525,000 square miles (1,400,000 km²), or 14.9% of the total area of the current United States. If the disputed western Texas claims are also included, that amounts to a total of 750,000 square miles (1,900,000 km²). If all of Texas is included, since Mexico had not previously acknowledged the loss of any part of Texas, the total area ceded under this treaty comes to 915,000 square miles (2,400,000 km²).

Considering the seizures, including all of Texas, Mexico lost 55% of its pre-1836 territory in the Treaty of Guadalupe Hidalgo.

for three centuries.

Subsequent organization and the North–South conflict

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Soon after the war started and long before negotiation of the new

• The Wilmot Proviso, which was created by Congressman David Wilmot, banning slavery in any new territory to be acquired from Mexico, not including Texas which had been annexed the previous year. Passed by the United States House of Representatives in August 1846 and February 1847 but not the Senate. Later an effort to attach the proviso to the Treaty of Guadalupe Hidalgo also failed.
• Failed amendments to the Wilmot Proviso by
  Las Vegas, Nevada, and Southern California, as well as any other territories that might be acquired from Mexico. The line was again proposed by the Nashville Convention
  of June 1850.
• Popular sovereignty, developed by Lewis Cass and Douglas as the eventual Democratic Party position, letting each territory decide whether to allow slavery.
• William L. Yancey's "Alabama Platform," endorsed by the Alabama and Georgia legislatures and by Democratic state conventions in Florida and Virginia
  , called for no restrictions on slavery in the territories either by the federal government or by territorial governments before statehood, opposition to any candidates supporting either the Wilmot Proviso or popular sovereignty, and federal legislation overruling Mexican anti-slavery laws.
• General
  organized territory
  , avoiding the question of slavery in the territories.
• The Mormons' proposal for a
  free states and the erosion of Southern parity in the Senate, while legitimizing the Church of Jesus Christ of Latter-day Saints
  .
• Senator
  34th parallel north
  .
• Senator
  Texas Panhandle), and the area to the south (including the southeastern part of today's New Mexico) would be divided at the Colorado River (Texas) into two slave states, balancing the admission of California and New Mexico as free states.^[4]
• First draft of the compromise of 1850: Texas's northwestern boundary would be a straight diagonal line from the Rio Grande 20 miles (30 km) north of El Paso to the Red River of the South at the 100th meridian west (the southwestern corner of today's Oklahoma).

• The
  Fugitive Slave Act
  was strengthened.

It quickly became apparent that the Mexican Cession did not include a feasible route for a transcontinental railroad connecting to a southern port. The topography of the New Mexico Territory included mountains that naturally directed any railroad extending from the southern Pacific coast northward, to Kansas City, St. Louis, or Chicago. Southerners, anxious for the business such a railroad would bring (and hoping to establish a slave state beachhead on the Pacific coast),^[5] agitated for the acquisition of railroad-friendly land at the expense of Mexico, thus bringing about the Gadsden Purchase of 1853.

• The
  Zimmermann Telegram, which partly offered Imperial German assistance to Mexico in returning a sizable portion of the Mexican Cession's southern territory, as well as the US state of Texas
  to Mexico in 1917.