Mexican-American War: Causes, Effects & Results

Instructor: David Lobb
The war with Mexico almost doubled the size of the United States, and opposition to the war demonstrated the growing sectional divide in America at the time. Explore the causes of the Mexican-American War and the effects it had on the growing nation.

Relations Between the U.S. and Mexico

By the time James Polk took office in 1845, relations with Mexico were already deteriorating. In fact, just two days after the inauguration in March, the Mexican Ambassador to the U.S. broke diplomatic relations and headed home to protest the annexation of Texas. The Mexican government was not happy with Polk's plan to grant Texas statehood and still debated the validity of the border boundary along the Rio Grande. In addition, Polk was encouraging the desires of many Californians to become part of America, and he ordered U.S. troops, under the leadership of General Zachary Taylor, into the disputed border area of Texas. After about a year of failed negotiations, Polk drafted a war message to Congress. That very evening, Polk received word that Mexican troops had attacked U.S. Troops inside Texas, which was now a U.S. state. Eleven Americans were killed, and many critics of Polk's message argued that he provoked the attack.

American map during the Mexican-American War
War map

The Road to War

The Mexican attack gave Polk the moral high ground; he argued that the U.S. would be responding to Mexican aggression and to the shedding of American blood on American soil. Congress quickly passed the war resolution, but support for the war was guarded. Congress authorized a call for at least 50,000 volunteers and some $10 million dollars to support the war effort, but opposition to the war effort was clearly seen along sectional lines. In the Mississippi Valley, where a fever to expand west ran high, so did support for the war; however, in New England, there was less enthusiasm for what was becoming referred to as 'Mr. Polk's War.' John Quincy Adams voted against participation and even labeled the conflict an 'unrighteous war.' A little-known, one-term Congressman from Illinois, Abraham Lincoln, led an effort challenging president Polk to show exactly where on American soil American blood had been shed. Other intellectuals from New England, such as Henry David Thoreau, openly protested the war, even going to jail for his opposition.

Many were ready to separate from the slave states, believing the war was about conquest; however, as the war moved forward, many had a change of heart, believing that the new territory would not be suited for big plantations and might actually serve their interest in bringing in free states. Manifest Destiny, or the argument that it was American destiny to expand across the continent, was a huge influence even to those who opposed the war.

Manifest Destiny was a major influence in the approval of the Mexican-American War.
Manifest destiny

The War

Both the U.S. and Mexico were ill prepared for war. In the months before, the country had twice flirted with war - with the British over border disputes in the Oregon Territory and with Mexico in the South. At the same time, nothing had been done to strengthen the armed forces. Despite the fact that American troops were poorly equipped and small in number, they outmatched the larger Mexican forces, which had their own issues with discipline, equipment, and training. Many of the Mexican troops had been forced into service or let out of prison to fight. Mexican cannons and gun powder were so faulty that American forces told of instances where they dodged Mexican cannon balls that would fall short and bounce on the ground.

The U.S. had no plan of action for the war. Zachary Taylor had scored two early victories and seemed to be the best choice to run the tactical war effort. Even before Taylor fought his first major battle in the North of Mexico, California and New Mexico had been taken. After receiving reinforcements, Taylor headed south into the heart of Mexico. With multiple fronts opened and fighting deep into Mexico, Mexican general Antonio López de Santa Anna was unable to stop the American push to the capital at Mexico City.

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