The Mexican Cession was the huge area of land that Mexico was forced to give up to the United States in 1848 at the end of the Mexican–American War, under the Treaty of Guadalupe Hidalgo.

Quick Scoop

What was the Mexican Cession?

  • It was the transfer of territory from Mexico to the U.S. after the Mexican–American War, formalized in 1848.
  • The deal was written into the Treaty of Guadalupe Hidalgo, signed in February 1848.
  • The U.S. paid Mexico about 15 million dollars and agreed to assume some debts owed to U.S. citizens.

What land are we talking about?

The Mexican Cession covered most of what we now think of as the American Southwest and West Coast.

Key areas included:

  • California and Nevada
  • Utah
  • Most of Arizona
  • Parts of New Mexico and Colorado
  • A small part of present‑day Wyoming

Altogether, Mexico lost about 55% of its pre‑1836 territory when you include Texas, which it also had to recognize as U.S. land.

Why did it happen?

  • The U.S. had been pushing a Manifest Destiny idea: that it was destined to expand across the continent.
  • After the U.S. annexed Texas in 1845, Mexico saw it as an act of aggression, and war broke out in 1846.
  • When U.S. forces captured Mexico City in 1847, Mexico had little choice but to negotiate and accept a harsh peace.

Under the treaty:

  • The border was set at the Rio Grande.
  • Mexico gave up claims to Texas and ceded the western lands (the Mexican Cession) to the U.S.

Why does it matter?

  • It instantly made the United States a continental power , stretching from the Atlantic to the Pacific.
  • The new territory intensified fights over whether slavery would spread west, helping set the stage for the American Civil War.
  • For Mexico, it was a massive loss of land and a lasting source of national trauma and resentment.

Mini timeline

  1. 1845 – U.S. annexes Texas; Mexico disputes the border.
  1. 1846–1848 – Mexican–American War.
  1. February 2, 1848 – Treaty of Guadalupe Hidalgo is signed.
  1. 1848 onward – Mexican Cession lands gradually become U.S. states such as California, Nevada, Utah, etc.

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The Mexican Cession was the vast territory Mexico ceded to the United States in 1848 after the Mexican–American War, under the Treaty of Guadalupe Hidalgo, reshaping both countries’ borders and futures.

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