How did the Mexican War and the idea of Manifest Destiny become divisive political issues?

Manifest Destiny was of course a key motivating factor for the Mexican War in that it claimed the right of the United States to claim territories that belonged to Mexico. But it was the war itself that was truly divisive. From its outset, many Northern Whigs charged that it was being fought on behalf of a "slave power" conspiracy that sought to expand slavery into the lands claimed as a result of the war. Once...

Manifest Destiny was of course a key motivating factor for the Mexican War in that it claimed the right of the United States to claim territories that belonged to Mexico. But it was the war itself that was truly divisive. From its outset, many Northern Whigs charged that it was being fought on behalf of a "slave power" conspiracy that sought to expand slavery into the lands claimed as a result of the war. Once it came to an end, a fierce political debate commenced over this very issue--would slavery be permitted in the new American Southwest and California? Southern politicians argued that it must be allowed, and Northern Whigs argued otherwise. The Compromise of 1850, which attempted to solve this issue, only sowed the seeds for more conflict by including a more robust Fugitive Slave Act that outraged even those Northerners who had not felt particularly strongly about slavery in the past. It is a bit too simplistic to say that Manifest Destiny was the cause of all this--many of its supporters, including John O'Sullivan, who coined the phrase in the first place, thought the territorial expansion of the United States would ultimately allow slavery to be abolished. But in the end the Mexican War was the result of Manifest Destiny, and fierce and ultimately divisive political conflict over slavery's future was the result of the Mexican War.