The United States did not take all of Mexico after the Mexican-American War primarily due to a combination of intense Mexican resistance, significant domestic political opposition within the U.S., and the immense logistical and financial burdens of occupying such a vast and populous territory.
While the U.S. Army achieved major military victories, occupying the entire nation proved to be an overwhelming challenge. The prospect of integrating millions of Mexicans into the Union also raised contentious issues that threatened to destabilize American politics.
Key Factors Preventing Full Annexation
Several critical factors converged to prevent the U.S. from annexing all of Mexico:
-
Intense Mexican Resistance and Guerrilla Warfare:
Even after major cities like Mexico City fell, the U.S. military faced relentless opposition across the country. American forces found themselves embroiled in widespread guerrilla warfare. For example, in regions like Tamaulipas, between Veracruz and Mexico City, U.S. army units encountered constant harassment and attacks. This made it virtually impossible for America to establish effective control over Mexican territory, as they were simultaneously losing men, supplies, and military equipment to these persistent, irregular forces. This ongoing conflict highlighted the impracticality and high cost of a full, long-term occupation. -
Domestic Political Opposition in the U.S.:
Perhaps the most significant barrier was the deep internal division within the United States, particularly concerning the issue of slavery.- Slavery Debate: Annexing all of Mexico, with its vast territory and diverse population, would have ignited an explosive debate over whether new states formed from these lands would be free or slave. This threatened to unravel the fragile balance between free and slave states, pushing the nation closer to civil war.
- Racial and Cultural Concerns: Many American politicians and citizens harbored racist views and were reluctant to incorporate a large, predominantly Catholic, Spanish-speaking, and mestizo population into the Union. They feared it would dilute American identity and create an unmanageable electorate.
- Democratic Principles: Some opposed annexation on moral or philosophical grounds, arguing that forcibly incorporating a large, unwilling population was antithetical to American principles of self-determination and democracy.
-
Financial and Logistical Burdens:
The cost of war was already substantial, but the expenses of occupying and administering all of Mexico indefinitely would have been astronomical.- Perpetual Occupation: Maintaining a large military presence to suppress ongoing resistance and govern a hostile population would have been a continuous drain on the national treasury and manpower.
- Administrative Challenges: Governing a foreign nation with a different legal system, language, and culture would have posed immense administrative complexities for which the U.S. had little precedent or capacity.
Summary of Constraints
The following table summarizes the primary reasons the U.S. chose not to annex all of Mexico:
| Factor | Description | Impact on Annexation |
|---|---|---|
| Mexican Resistance | Widespread guerrilla warfare (e.g., in Tamaulipas, between Veracruz and Mexico City) significantly hampered U.S. efforts to establish control, leading to continuous attrition of troops and resources. | Made military occupation economically unsustainable and militarily untenable. |
| Domestic U.S. Politics | Intense sectional conflict over the expansion of slavery into new territories; widespread racial and cultural prejudice against incorporating a large, non-white, Catholic population. | Created insurmountable political division and strong opposition to large-scale annexation. |
| Financial & Logistical Cost | The prohibitive expense of maintaining a long-term military occupation and the immense challenges of administering a vast, culturally distinct, and resistant nation. | Deemed too costly and impractical for the U.S. to undertake without severe domestic consequences. |
Ultimately, the U.S. settled for a treaty that granted it California, Nevada, Utah, Arizona, New Mexico, and parts of Colorado and Wyoming—territories that were sparsely populated and less likely to provoke immediate sectional crises than the more populous central regions of Mexico.
