How the Mexican-American War Impacted the Slavery Debate.

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The war where the border was moved.

The Mexican-American War was a war between Mexico and the United States of America between April 24, 1846, and February 2, 1848.

After the introduction of centralist law in Mexico, Texas had declared independence in 1835. Mexican president-dictator General Antonio López de Santa Anna marched on to Texas with an army.

After first capturing the Alamo, De Santa Anna was defeated at the Battle of San Jacinto. De Santa Anna signed a treaty in which he recognized Texas’ independence.

The Mexican government was furious with De Santa Anna and refused to recognize the treaty.

In 1845, Texas was admitted to the US as a state, much to Mexico’s anger. The Nueces River formed Texas’s southern border. The US had recognized this in 1819 in the Adams-Oní Treaty with Spain.

With that treaty, Spain renounced Florida, and the treaty then established the borders between the US and Spanish possessions. But the US government now claimed the entire area up to the Rio Grande for Texas. Meanwhile, the US government tried to buy all of northern Mexico, but the Mexicans refused to sell the area.

When the US government realized that Mexico would not cede the areas, President James K. Polk (D-Tennessee) sent General Zachary Taylor to the Nueces and the Rio Grande area. On April 25, 1846, a battle broke out, the Thornton Affair, in which 16 Americans and an unknown number of Mexicans were killed.

After this, President Polk made a flaming speech in Congress,

“Mexico has crossed the United States border, invaded our territory and spilled American blood on American soil.”

Congressman Abraham Lincoln (W-Illinois)was the only one to contest this claim based on the aforementioned 1819 treaty and has since been nicknamed “honest Abe.” On May 13, Congress declared war on Mexico, and ten days later, Mexico declared war on the United States.

The course of the war

Admiral John D. Sloat was sent to Mexican Alta California, California, where American colonists had proclaimed California’s independent republic.

Sloat helped the Californians dislodge the Mexican army. In the meantime, Stephen W. Kearny conquered New Mexico. After that, San Diego and Los Angeles were also occupied.

General Zachary Taylor had now crossed the Rio Grande. He narrowly defeated a Mexican army at the Battle of Monterrey in September.

Santa Anna, who had seized power in Mexico, marched north against Taylor with an army of 20,000 but was defeated at the Battle of Buena Vista. The US government asked Mexico to surrender, but Mexico refused.

General Winfield Scott then landed a US Army at Vera Cruz. He managed to take the city and marched on to Mexico City.

There was heavy fighting around Mexico City during battles at Contreras, Churubusco, Molino del Rey, and Chapultepec. The “Niños Héroes” were killed in this last battle. They became immortal to Mexico. On September 15, 1847, the last Mexican resistance was broken.

On February 2, 1848, Guadalupe Hidalgo’s Peace was signed, ending the war and giving a third of Mexico’s territory to the United States. The war had claimed the lives of 25,000 Mexicans and 13,000 Americans.

The Aftermath

This war brought the border between the US and Mexico further south. In Mexico, the war led to a deep distrust of the gringos.

In the north of the US, people were not too positive about this war. It was seen as an attempt by the Southern states to increase the area where slavery was allowed.

Ironically, slavery was never allowed in most of the territory conquered in the Mexican War. In 1850, California, the largest and richest state conquered in the Mexican War entered the union as a free state. Southern anger at the seizure of the new territories by antislavery northerners became one cause of the Civil War.

In the United States, this war is sometimes known as the Mexican War. On the other hand, in Mexico, the war is commonly referred to as Guerra de Intervención Norteamericana, War of North American Intervention. This difference in the name makes it clear that both countries view the war differently today.

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