Reasons why the United States didn’t annex Mexico following the Mexican-American War

Historically speaking, the Mexican-American War, which occurred from 1846 to 1848, was the first major U.S. military combat conducted on foreign soil. It was fought between an ideologically divided and logistically unprepared Mexico and the  expansionist administration of U.S. President James K. Polk.

President Polk and his supporters, mainly pro-slavery southerners in the Democratic Party, felt the United States had a “Manifest Destiny” to expand its territories to the Pacific Ocean.

The United States was a territory-hungry nation and bent on capturing Mexico with the aid of Polk’s influence and the brute commanding nature of General Zachary Taylor, who later became the 12th President of the United States.

The United States easily won the two-year war due to Mexico’s ill preparedness. Washington was able to annex more than 520,000 square miles (or 1,300,000 square km) of Mexico’s land. The additional territory increased the size of the United States by about one-third.

Considering just how convincing U.S. victory was over Mexico, the question that begs to be answered is: Why didn’t the United States annex all of Mexico following the war?

Quick Summary of the Mexican-American War

Why didn’t the U.S. annex all of Mexico

The Mexican-American War was a classic case of a superior nation bullying a weaker nation into utter submission. This point is reflected in U.S. general and later 17th President of the United States U.S. Grant’s assertion that the war was “one of the most unjust ever waged by a stronger against a weaker nation”.

Annexation of Texas happened under a James K. Polk administration that wholeheartedly held the view that it was the U.S.’s destiny to expand its borders all the way to the Pacific Ocean. This view was known back then as the Manifest Destiny.

Sincere attempts were made on both sides to resolve the border dispute. When negotiations broke down, then-U.S. President James K. Polk, an ardent supporter of Manifest Destiny, sent American troops into the territories around the Rio Grande. Perhaps Polk was luring the Mexicans into a confrontation.

It must be noted that it was not all lawmakers on Capitol Hill that favored a war with Mexico. The Whig Party and many anti-slavery campaigners, including famous abolitionist Frederick Douglass, opposed Polk’s confrontational stance against Mexico. Opponents of the war held the view that Polk and his pro-slave southerners simply wanted to annex territories so as to extend slavery.

Anti-slavery advocate Frederick Douglass was one of the leading voices that opposed the Mexican–American War

Seeing the presence of American troops on their soil as blatantly aggressive, Mexican troops attacked U.S. troops at the Rio Grande on April 25, 1846. The attack left about a dozen U.S. troops dead, while 16 were injured.

In spite of those vocal voices against the war, Polk was able to get Congress to declare war on Mexico on May 13, 1846. About 70,000 U.S. forces were involved in the two-year war. Mexico’s ill-prepared and relatively untrained forces of about 80,000 proved to be no match for the United States.

With strong military leadership from the likes of General Zachary Taylor and Winfield Scot, the United States was able to secure a resounding victory over her neighbor. By the middle of September 1847, the U.S. had successfully overrun Mexican forces and captured Mexico City.

On February 2, 1848, Mexico and the United States signed a peace accord – the Treaty of Guadalupe Hidalgo. The U.S. emerged with about 520,000 square miles (1,300,00 square km) of Mexico’s territory for the price of $15 million. The ceded territories include present-day U.S. states of Nevada, Utah, California, Arizona, Texas, and parts of Colorado.

Supporters of a complete annexation of Mexico

The biggest dilemma of President Polk after the war was how much land he would seize from Mexico. The president had entered the White House on a campaign promise to expand the United States westward all the way to the Pacific.

Therefore, annexing all of Mexico, or a significant part of Mexico would have been in keeping up with Polk’s election campaign promise. It would also have favored pro-slavery politicians in the Democratic Party. Those politicians eyed the possibility of expanding slavery into those annexed territories.

The president’s advisors that favored full annexation suggested that the U.S. could crush any form of opposition by using a similar technique to the one used in dealing with the Native Americans.

James K. Polk and his pro-slavery supporters from the Democratic Party were very much in favor of annexing all of Texas. This was in keeping up with their Manifest Destiny policy.

Also, those in favor of full annexation drew the president’s attention to all the financial gains the U.S. could rake in, especially the mineral resources in southern Mexico. The ports in those areas were seen as opportunities to further expand the United States’ reach.

Polk tried to convince Congress that that annexing the whole of Mexico would offset the financial input he made during the war. However, try as they might, Polk and his pro-slavery politicians simply could not get the United States to annex the whole of Mexico.

Below, WHE presents some of the major reasons why the United States did not annex all of Mexico:

Why didn’t the United States annex Mexico?

There were a significant number of lawmakers in Washington, D.C. that opposed full annexation of Mexico. Even within Polk’s administration, some officials kicked against the annexing of all of Mexico. Those politicians cited some of the following reasons:

The first disagreement covered issue of slavery. Unlike the United States, Mexico had already abolished slavery a decade before the war broke out. The question that was then asked was: would those annexed Mexican territories be slave states or a free states?

Polk, who was in favor of of Mexico being a slave state, had to contend with harsh opposition since the House of Representatives was controlled by the anti-slavery-dominated Whig Party. With his Democratic Party as the minority, the president simply had to reach a compromise deal with the Whigs, who staunchly opposed having more slave states in the Union.

Another reason why the Polk administration was hesitant to annex all of Mexico had to do with the dire consequence that could have followed. The president was advised that Mexicans in those annexed territories could mount a Guerrilla warfare and even fight to the bitter end. This would make America’s stay in the country very unbearable. In the long run, that instability could snuff out any economic or political benefits of a full annexation.

The very thorny issue of racism came to the fore during talks about annexing Mexico. It so happened that a reasonable of US politicians raised concern that were all of Mexico annexed, the U.S. would struggle to handle the sudden influx of people of different cultures, i.e. Spanish-speaking and non-whites.

Plans for full annexation couldn’t come to fruition because of Nicholas Trist, who was known to have negotiated the Treaty of Guadalupe Hidalgo in 1848, which prevented the U.S. from annexing all of Mexico. Nicholas Trist – Portrait by American painter John Neagle, in 1835

Perhaps the biggest wrench in the plans of full annexation of Mexico came from Nicholas Trist, the American lawyer and diplomat who served as the chief negotiator of the Treaty of Guadalupe Hidalgo. Polk had sent the Trist – a chief clerk in the State department – to negotiate for the entirety of Mexican territories. Trist, however, had other ideas.

So, when the president caught wind of Trist’s slight pro-Mexican plans, Trist was fired. Rather than return to the States, Trist defied Polk’s order and went ahead to negotiate and sign a lesser settlement treaty with the Mexicans. It’s been said that Trist believed the U.S. would have disastrous outcomes if it tried to be a republic and an empire at the same time.

Polk hoped the treaty brought in by Trist would be rejected, however, he later changed his mind, as he did not want the issue to drag on any further. The president then reluctantly submitted the the treaty to lawmakers on Capitol Hill.

The U.S. Senate gave its consent (by a vote of 34 to 14) to the Trist-negotiated treaty on March 10, 1848. The United States removed its troops from Mexico City, and the issue of annexing Mexico completely came to a close. The U.S. had to settle for a “meagre” 520,000 square miles of their neighbor’s territory.

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