The Reasons of America Coming to War with Mexico Between 1846 to 1848

From 1846 to 1848, the United States of America and Mexico went to war. There were several reasons why they did so, three of the most important ones were manifest destiny, the annexation of Texas, and Slidell’s mission. Americans gradually had been moving westward for two centuries, but in the 1830s and 1840s, they pushed across the continent.

The Americans in Texas greatly outnumbered the native Mexicans, and they sought full statehood for the province in order to gain home rule. Polk appointed John Slidell of Louisiana as minister to Mexico and instructed him to offer up to 30 million dollars to settle the disputed claims and purchase California and New Mexico—the territory between Texas and California. Because of the manifest destiny, the annexation of Texas, and Slidell’s mission the president informed his cabinet that the U.S. “had ample cause of war.”

The 1840s were years of extraordinary territorial growth for the United States. During a four year period, the national domain increased by 1.2 million square miles, a gain of more than sixty percent. So rapid and dramatic was the process of territorial expansion, that it came to be seen as an inexorable process, prompting many Americans to insist that their nation had a "manifest destiny" to dominate the continent. The idea of Manifest Destiny held that Americans were superior to most other people in a number of ways. They were said to have a superior form of government, a superior culture, and a superior religion. For these reasons, it was said, they were destined by God to expand their territory. This attitude led to the westward migration.

First, it helped lead to the expansion of US territory. It helped lead to the Mexican-American War because it promoted the attitude that America deserved all of that land more than the Mexicans did. America’s God-given destiny made it acceptable to take the land from Mexico. The same attitude applied to the Indians as American settlers moved west. The idea was that the Indians were so inferior that they did not deserve the use of the land. They were to be moved and the land was to go to the Americans who deserved it and would use it well. The spirit of "Manifest Destiny" pervaded the United States during the Age of Reform—the decades prior to the Civil War. John L. O’Sullivan, the editor of the influential United States Magazine and Democratic Review, gave the expansionist movement its name in 1845 when he wrote that it is "the fulfillment of our manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions."

Manifest Destiny was stimulated by nationalism and an idealistic vision of human perfectibility. It was America’s duty to extend liberty and democratic institutions across the continent. Texas gained its independence from Mexico in 1836. Initially, the United States declined to incorporate it into the union, largely because northern political interests were against the addition of a new slave state. Nonetheless, annexation procedures were quickly initiated after the 1844 election of Polk, who campaigned that Texas should be “re-annexed” and that the Oregon Territory should be “re-occupied.” Polk also had his eyes on California, New Mexico and the rest of what is today the U.S. Southwest. When his offer to purchase those lands was rejected, he instigated a fight by moving troops into a disputed zone between the Rio Grande and Nueces River that both countries had previously recognized as part of the Mexican state of Coahuila. From Mexico’s point of view, the annexation of Texas to the United States was inadmissible for both legal and security reasons. Thus, when the Mexican government learned of the treaty signed between Texas and the United States in April 1844, it reaffirmed the posture it had expressed a year before that Mexico would consider such an act "a declaration of war." And later, when the Congress approved the joint resolution inviting Texas to join the United States, Mexico suspended diplomatic relations with its neighbor. Mexico asserted that the annexation of Texas—whether by treaty or in a U.S. Congressional resolution—was a violation of the 1828 border treaty, which had acknowledged Mexico’s sovereignty over that territory.

Consequently, such acts were a violation of the fundamental principles of international law, and furthermore, they established a dangerous precedent threatening Mexico’s territorial security, since the same formulas could be used to annex other areas along the border.Mexico severed relations with the United States in March 1845, shortly after the U.S. annexation of Texas. In September U.S. Pres. James K. Polk sent John Slidell on a secret mission to Mexico City to negotiate the disputed Texas border, settle U.S. claims against Mexico, and purchase New Mexico and California for up to $30 million. Mexican Pres. José Joaquín Herrera, aware in advance of Slidell’s intention of dismembering the country, refused to receive him. Slidell hinted to Polk that the Mexican reluctance to negotiate might require a show of military force by the United States. Under the guidance of General Zachary Taylor, U.S. troops were stationed in the disputed area between the Nueces and the Rio Grande, ready to defend against Mexican attack.

On May 9, 1846, Polk began to prepare a war message to Congress, justifying hostilities on the grounds of Mexican refusal to pay U.S. claims and refusal to negotiate with Slidell. That evening he received word that Mexican troops had crossed the Rio Grande on April 25 and attacked Taylor’s troops, killing or injuring 16 of them. In his quickly revised war message—delivered to Congress on May 11—Polk claimed that Mexico had “invaded our territory and shed American blood on American soil.” Congress responded with a war resolution and an authorization for 50,000 volunteers on Mexico on May 13, 1846.

In conclusion, the United States of America went to war with Mexico. There were several reasons why they did so, thee of the reasons were manifest destiny, the annexation of Texas, and Slidell’s mission. Americans gradually had been moving westward for two centuries, but in the 1830s and 1840s, they pushed all the way across the continent. The Americans in Texas greatly outnumbered the native Mexicans, and they sought full statehood for the province in order to gain home rule. Polk appointed John Slidell of Louisiana as minister to Mexico and instructed him to offer up to 30 million dollars to settle the disputed claims and purchase California and New Mexico, the territory between Texas and California. Because of the manifest destiny, the annexation of Texas, and Slidell’s mission the United States had more than enough reason to go to war with Mexico.