The Mexican-American War: Photos from the 1800s show a young nation's conflict

Prominent American Civil War Generals Robert E. Lee, Ulysses S. Grant, and Stonewall Jackson were among those who served in the war against Mexico.

In the month of May, two key anniversaries that spurred the Civil War into motion occurred between the United States and Mexico over a conflict that centered on Texas. 

The Mexican-American War became the first large-scale success of a U.S. military force on foreign soil, according to the National Constitution Center.

The independent Republic of Texas established its independence from Mexico a decade before joining the U.S. as part of the nation’s embracing of Manifest Destiny.

President James K. Polk saw the Union’s acquisition of Texas, California, Oregon and seven other territories as spreading democracy over the continent. Mexico considered the annexation of Texas as an act of war. 

After a series of border skirmishes and a failed offer to buy Texas and California from Mexico, on May 8, 1846, U.S. and Mexican troops clashed on the prairie of Palo Alto, Texas — the first battle in a two-year long war that changed the map of North America.

President Polk requested the U.S. Congress to go to war, which was declared on May 13, 1846. 

The mostly-volunteer U.S. military secured control of Mexico after a series of battles, and the Treaty of Guadalupe Hidalgo was signed on February 2, 1848. Two years after the war had begun, on May 26, 1848, both sides ratified the peace treaty that ended the conflict, and the territorial size of Mexico was cut in half.

The pact set a border between Texas and Mexico and ceded California, Nevada, Utah, New Mexico, most of Arizona and Colorado, and parts of Oklahoma, Kansas, and Wyoming to the U.S.

The acquisition of this expansive territory with the issue of slavery unresolved "lit the fuse that eventually set off the Civil War in 1861," according to the Palo Alto Battlefield National Historical Park.

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