Why Mexican Officials Opposed the U.S. Annexation of Texas

Mexico's outrage over the 1845 U.S. annexation of Texas stemmed from five interlocking grievances:

1. Violation of Territorial Integrity

Texas had been Mexican territory since 1821. Mexico never recognized the 1836 Texas Republic's independence; it viewed the region as a rebellious province. U.S. annexation was thus an act of aggression—seizing sovereign Mexican land.

2. Breach of the 1828 Treaty of Limits

The U.S.–Mexico Treaty of Limits (1828) confirmed the Sabine River as the boundary and affirmed Mexican sovereignty over Texas. Annexation unilaterally abrogated this treaty.

3. Fear of U.S. Expansionism

Mexican leaders (correctly) saw Texas as a stepping stone. President Polk's "Manifest Destiny" rhetoric and desire for Pacific ports (California, New Mexico) signaled further territorial ambitions.

4. Loss of Northern Frontier Control

Annexation emboldened American settlers to push into Nuevo México, Alta California, and Coahuila, weakening Mexico's tenuous hold on its northern departments.

5. Diplomatic Rupture

Mexico severed diplomatic relations with the U.S. in March 1845. Polk's subsequent troop deployment to the disputed Nueces Strip (between Nueces River and Rio Grande) precipitated the Mexican–American War (1846–48), which cost Mexico >50% of its territory.

In Mexican memory, the annexation remains the original sin of U.S. imperialism in North America.