In the US, prior to the Mexican American war, the Mason Dixon line divided the slave holding states from the free states of America. It is easy to believe that the issue of slavery was one that pervaded all cultures and nations in the mid 1800's, but it is important to remember that Mexico did not partake in slavery. This meant that the more land the Mexican gave up, the more territory the US would have in the expansion of slavery. One of the most vocal, anti-war advocates was Frederick Douglass who criticized both political parties in his article published by his newspaper The North Star. Douglass writes that it is ironic that the United States considers its ventures in expansion synonymous with extending the area of freedom and yet bring slavery and rampant inequality into new territories as well. He identifies James K. Polk as a "slaveholder" which directly contrasts with his other name of "friend of freedom". Clearly, Douglass has a poignant connection to the issue of slavery because he was a freed slave who worked to educate and employ himself, but Douglass and other freed slaves were not the only vocal proponents of anti-war movement.
Clergymen and reformers in and around New England were also in opposition to the US war with Mexico on the principle of equality. The New England area was a hotbed for anti-war advocates where communities of Unitarians, Quakers, and abolitionist feminists would spread their anti-war ideology and work in tandem to protest the war efforts. The anti-war movement's ranks were filled with such a diverse cast of characters that all had such far reaching networks that demographics of all kinds were affected by the anti-war ideology in a number of ways. Feminist and abolitionist Lucretia Mott spread her anti-war message by giving her address to a group of women's rights activists. Clergyman Theodore Parker spread his anti-slavery ideals by mixing it with theology when claiming, "Teach your rules that you are Americans, not slaves; Christians, not heathen; men, not murderers, to kill for hire!" Parker extends his commentary to criticize not only slave holders, but all so-called Christians that consider people of other races to be heathens i.e. Native Americans and African Americans.
When contrasting the anti-war proponents' urges for peace, equality, and a cessation of the expansion of slavery with war advocates' thirst for more land and property and the allowance to own people to work all of that new land does it become clear that the line in the sand was becoming more and more defined as Americans realized how far slave holders would go in order to expand their lifestyles westward. For the sake of obtaining more land for Americans, President Polk withdrew articles from the Treaty of Guadalupe-Hidalgo that would reserve land grants for people who had been given land from the Mexican government. This blatant disregard for Mexican rights shows so clearly how little the US government cared for members of other races and former nationalities.
The greater impact of understanding the relation between racism and the US-Mexican war is this: the stark ideological differences that existed between the slave holding, prejudiced, land grabbing states and the abolitionist North created two very opposite and opposing identities. The issue of slavery in Texas was a complex one, and with every step either to or away from slavery that this new state took, the more imminent the American Civil War would seem. Slavery was quickly transitioning from a hot button issue to a problem that could no longer be touched without raising burning questions. The division between slave and free states could no longer exist in a country that was divided on the issue. The fugitive slave laws that existed in the North forced free states to decide whether or not they would condone the act of slave holding, and the American-Mexican War showed the North that they could no longer stand idly by. The South also had reached it limit in being shorted of their "property" by Northerners that did not abide by the fugitive slave laws. This culmination of contrasting identities, Texas slave disputes, and rising tensions would all give way to another conflict that would occur only a handful of years later: the American Civil War.