The Mexican-American War 1846-1848

a.k.a. the U.S.-Mexican War

(+ 1845 annexation of Texas)

One of "America's Forgotten Wars," the Mexican-American War often fades in the shadow of the American Civil War two decades later (1861-65), which remains a defining event for Americans interested in history, partly because of continuing internal-domestic political, regional, and racial divisions.

If many Anglo-Americans forget the Mexican-American War, citizens and historians of Mexico don't forget the Mexican-American War.

  • Prior to the establishment of the Republic of Texas in 1836, Mexican territory comprised 1.7 million square miles

  • By 1848 Texan Independence, the Mexican-American War, and related conflicts had reduced that territory to under 800,000 square miles—a loss of more than half of Mexico's territory. (Indian nations in ceded territories included the Hopi, Navajo, Zuni, Apache & many others.)

The Mexican cession to the USA constituted

  • the entire territory of new states of California, Utah, New Mexico, & Nevada;

  • most of Arizona and Colorado, and parts of Texas, Oklahoma, Kansas & Wyoming. (TX War of Independence 1835-6)

In return for these land cessions the USA paid Mexico $18 million and assumed $3 million in debts owed by Mexico to U.S. citizens.

thanks to

Mexico in 1786

Additional impacts on Mexico, USA, and shared continent:

  • As with U.S. justifications for conquest of Indian territory, war hawks argued that New Spain's small population and lack of development created a natural vacuum that American western migration must fill. In the early 1800s, birth and survival rates for Europeans and European Americans were rising, generating migration to less-populated territories.

  • In response to this demographic motivation, after the war the Mexican government encouraged large families and population growth to discourage further U.S. invasions.

  • In the twentieth century, rapidly increasing population growth esp. among poor Mexicans fueled immigration to the United States, whose native birth rates dropped after the 1960s.

  • Thus both the U.S. invasion and conquest of Mexico in the 1800s and Mexican immigration to the U.S. in the 1900s & 2000s were driven by population dynamics, particularly the early stages of the Demographic Transition. (Mexican birthrates are gradually declining. Birth rates for immigrants to the USA typically decline after a generation or two of assimilation.)

Why did the U.S. fight the Mexican-American War?

As with the Iraq and Afghanistan Wars, the Mexican-American War was a "voluntary war"—Mexico posed no direct threat to American security.

The USA's justification for national expansion was its "manifest destiny" to spread democratic capitalism and gain natural resources "from sea to shining sea." This goal became American policy under the Presidency of James K. Polk (1845-49) and was largely accomplished by the Mexican-American War.

Further impact of Mexican-American War on U.S. history:

The Mexican-American War led directly to the American Civil War: the war's expansion of U.S. territories forced decisions over whether new states would be slave or free, increasing tensions and conflict between North and South.

  • Before annexation of Texas and the Mexican-American War, Americans who opposed slavery could rationalize that the South would eventually follow other modern societies in freeing its slaves—that is, slavery would die a natural death.

  • When Texas joined the USA as a slave state in 1845, this gradualist rationale fell apart—instead of dying, slavery was growing. Annexation of new states from the Mexican-American War escalated national tensions over the expansion or limitation of slavery until the Civil War broke out.

  • Many military leaders in the U.S. Civil War fought in the Mexican-American War, including Robert E. Lee and Ulysses S. Grant. Grant later denounced the Mexican-American war for its voluntary aggression (compare Iraq War) and for leading to the U.S. Civil War.

Other web sources:

Mexican-American War at the

PBS materials on the Mexican-American War

Discussion question(s):

How does knowledge of the Mexican-American War potentially change attitudes toward Mexican immigration?

How does knowledge of the Mexican-American War potentially change teaching of the U.S. Civil War?

thanks to BritannicaKids:

thanks to

Mexican War for Independence

Mexican Revolution