The Mexican-American War, which took place between 1846 and 1848, during the presidency of James Knox Polk, happened because Mexico refused to accept Texas´ petition to be accepted as independent from the country and they also refused to give up their claim over California and New Mexico. When this happened, the U.S deployed Zachary Taylor and men to the Rio Grande, which was then expected by the U.S to be accepted by Mexico as the new border between them and also sent representatives to Mexico City to negotiate. However the Mexican government refused and attacked American forces. This led to a great confrontation and the loss of these states and almost Mexico City itself. Many people opposed this war on several grounds; two such groups were the Conscience Whigs and also the Transcendentalists. However, there were also people in favor of this conflict; one such group were the southern expansionists.
First of all, the Conscience Whigs, a group of northern Americans who broke away from the main Whigs party because they opposed slavery, argued against the Mexican-American War because they believed that this conflict would only serve to expand slavery into the newly acquired territory. This would also upset the existing fragile balance between the free and slave states and they did not want that either.
The Trascendentalists, a group born in New England from Ralph Waldo Emerson, and who followed the ideals set by trascendentalism of a person´s capacity to self-govern and self-direct and the rejection to the rule by the majority, rejected the Mexican-American War on the grounds that they believed the U.S government should not try to impose its system and beliefs on others, especially the Mexicans.
Finally, there were also those who thought the war was correct and should happen. These were the southern expansionists. Their reasons were first, that in this way they would be able to acquire more land and thus expand and gain better lands and resources and second, they would be able to ascertain that European powers like Britain, would be kept away from their borders and would not seek to attack them because of their power.
