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What caused the Mexican-American War between the United States and Mexico?
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Mexican-American War
Learn how the Mexican-American War ultimately pushed the United States closer to civil war.
The Mexican-American War (1846–1848) was primarily caused by the United States' annexation of the Republic of Texas in 1845 and a fundamental disagreement over the border between Texas and Mexico.12
Key factors contributing to the conflict included:
- Border Dispute: The United States claimed the border was at the Rio Grande, while Mexico insisted it was at the Nueces River, located 150 miles to the northeast.13
- Expansionist Ambitions: Driven by the concept of Manifest Destiny, U.S. President James K. Polk sought to expand U.S. territory to the Pacific Ocean. When diplomatic efforts to purchase California and New Mexico failed, Polk sought a rationale for military action.425
- Diplomatic Breakdown: Following the annexation of Texas, Mexico severed diplomatic relations with the U.S. and refused to negotiate with U.S. envoy John Slidell.25
- Military Skirmish: In April 1846, after U.S. troops were ordered into the disputed territory, a clash occurred between Mexican and American forces. Polk used this event to claim that Mexico had "invaded our territory and shed American blood on American soil," securing congressional support for war.435