Why did Mexico lose half its territory?

How and why did Mexico lose more than two million and 500,000 square kilometers of its territory?

Why did Mexico lose half its territory?
Texas trouble. Image: Public domain

Mexico was born to an independent life with the territorial extension that New Spain had, which is to say with approximately 4 million 400 thousand square kilometers in 1821. From that year until 1853, it lost more than two million, 500 thousand square kilometers: In 1836, Texas; between 1846-1848, practically half the territory, and in 1853, La Mesilla, almost 80 square kilometers further south of the states of Arizona and New Mexico.


- Mexican immigration policy, together with the expansionist zeal of the United States of America, are two of the main causes of the Mexican-American war.

- After Mexico's independence, the country was deeply worn out after eleven years of intensive war.

- Mexico after independence was in economic crisis, confusion and fear. While the United States was a country with a flourishing economy.

- Since colonial rule, and even after Independence, the Mexican government had to push for the colonization of the vast northern territories, including the Californias, New Mexico, and Texas, whose total population did not exceed 50,000 Mexican citizens.

- A colonization policy was proposed which consisted of selling land under certain conditions to improve the country's economy.

- A large number of people from other countries settled on the fertile plains of Texas and became legal citizens, but there were also multitudes of U.S. citizens who accepted the conditions.

- Relations between Mexico and the United States during this period are marked by U.S. territorial expansionism.

- Minister Joel R. Poinsett left no doubt about the expansionist appetites that sought to annex the province of Texas, citing as evidence the Louisiana treaty of sale, including all of that Mexican territory.

- Mexico's position is forceful: only the limits of the 1819 Adams-Onís Treaty, which indicated the territorial limits between the territory of New Spain and the United States, will be accepted. In response, the U.S. government collaborates with the Texan majority that wants to become independent of Mexico and a new state of the United States.

- The following year, Lucas Alamán enacted a Colonization Act, by which he intended to hinder the massive arrival of U.S. citizens in Texas.

- In 1836 a multitude of Texas independentistas declared themselves against the dictatorship of Antonio López de Santa Anna, declaring also the independence of Texas. The Mexican response could not be other than to eliminate the rebels and force the province to continue inside Mexico.

- From 1836 to 1845 Texas would be governed as an independent republic, and Mexico would consider it a renegade province.

- The withdrawal of the Mexican army did not consolidate the existence of a clear border between Texas and Mexico.

The Mexico-United States War

- The U.S. Congress voted for the annexation of Texas in late February 1845.

- Two political groups were to be formed in Texas: a small party for Texas independence, and a larger and more popular party, headed by Samuel Houston, in favor of annexation to the United States.

- In early 1846, U.S. President James Polk ordered troops to advance south of Rio Grande into disputed territory on the Mexican border.

- On May 3, 1846, the Siege of the Texas Fortress was initiated when a Mexican artillery contingent opened fire on Fort Texas.

- Declaration of War: The U.S. Congress declared war on May 13, 1846, while Mexico declared war ten days later on May 23, 1846.

- Mexico is defeated after the fall of Chapultepec which had two consequences: the U.S. occupation of Mexico City and Santa Anna's new resignation as president of the nation.

Treaty of Guadalupe - Hidalgo

The U.S.-Mexico war ended on February 2, 1848, with the signing of the Treaty of Guadalupe Hidalgo, then entitled "Treaty of Peace, Friendship, Limits and Final Settlement between the United Mexican States and the United States of America.

The treaty declared peace between the two nations, lifted the blockade of Mexican ports, agreed on the repatriation of prisoners of war, delimited the border between the two nations from the Gulf of Mexico to the Pacific Ocean, passing through the Gila River, the Rio Bravo, the Colorado River, and the borders of New Mexico and Upper California.

In addition, Mexicans relinquished their claims to Texas and ceded to the United States territories of the current states of Arizona, California, Nevada, Utah, New Mexico, and parts of Colorado, Wyoming, Kansas, and Oklahoma.

Causes of Mexico's defeat

- The largest U.S. population - 20,000,000 vs. 9,000,000 people in Mexico

- Peace - Except for the War of 1812, the United States had a period of peace and political stability that lasted many years. In Mexico, internal political conflicts were always constant.

- The superiority of U.S. weapons, ammunition, quality of weapons, factories, and little support from the Mexican federation.

The signing of the Treaty

- The Treaty of La Mesilla was signed on December 30, 1853. This treaty, which in the United States of America is known as the "Gadsden Purchase".

- Santa Anna was willing to negotiate because he needed money to rebuild the armed forces to defend himself from the United States. Gadsden realized that Santa Anna needed the money and passed this information on to his government.


- The acquisition was enough to produce the anger of the Mexican people, who saw Santa Anna's actions as yet another act of treason. The sale of La Mesilla would put an end to General Santa Anna's political career.

- New purchase offer

- On March 4, 1857, James Buchanan assumed the office of President of the United States and reported his projects to John Forsyth, U.S. Ambassador to Mexico City.

- In 1857 he sent him instructions to buy the peninsula of Baja California and part of the territory of Sonora and Chihuahua, until the 30th parallel.

- Secretary of State Lewis Cass gave Forsyth suggestions on how he could convince President Ignacio Comonfort to agree to his proposals.

- On January 21, 1858, Benito Juarez became president of Mexico.