The Mexican-American War was one of the wars in American history that could be called a war of expansion. To some people this war was truly unavoidable. There are many causes for the Mexican-American War, but the two basic causes were the U.S. policy of Manifest Destiny and the annexation of Texas. By the time Mexico gained its independence from Spain, Americans were already on the border of the new nation. But in Mexico's mine they were already over the border.
The year before the Mexican-American War began, John O"Sullivan, editor of the Democratic Review, used the phrase Manifest Destiny to describe the thought of stretching the U.S. from coast to coast. Elected into office in 1845, the ever so aggressive President Polk desired California and would go to any measure to acquire the Mexican Territory. Polk believed heavily in Manifest Destiny. The passion the U.S possessed for expansion dated back to the 1830s when they offered Mexico $5million and then $25million for California. Mexico refused the offers both times. The refusal fueled the U.S. to want to expand its land even more. .
In Mexico, tension stirred between the newly settled Texans and the Mexican government. Many of the Texans were unhappy with the way that the government in Mexico City tried to rule them from such a distance. Most Texans practiced religions of Southern America, such as Southern Baptist or Methodist. The Native Mexicans were Roman Catholic, which posed a problem within the church and state. There were also other cultural differences, such as the Texans learning English when the agreement for them to live in a Mexican province was for them to learn Spanish. There were other reasons for reasons were contributors to the revolution of Texas. Although Texans did claim their independence in March of 1836, there was still disruption in the Texan government. The most important factor to the tension between Texas and Mexico was that the United States began to have sympathy for those who were born on Texan soil.