I’m not suggesting that they would have automatically been accepted as states. But as territories, I’m only suggesting that it was the South’s assumption that they’d eventually become slave territories. Since we ended up with the territories after the war, and they weren’t made into slave territories, they were obviously wrong. That doesn’t mean that it wasn’t just a land grab for the purposes of expanding slavery. The funny thing is, I was defending the South as not being “the bad guy” in the civil war in another thread, but I can squarely blame them for making us the bad guy in this war.
The Mason-Dixon Line had nothing to do with slavery…It served as the resolution of a boundary dispute between the colonies of Pennsylvania and Maryland. I believe you intended to cite the Missouri Compromise and latitude 36°30′ north. Also, any new state below that line would not “automatically become a slave state.” Being below that line meant a territory could decide to join the Union as a state that allowed slavery. California is a perfect example of a state that is located below 36°30′ north that voted to enter the Union as a “free state.”
An obscure one-term Congressman from Illinois opposed the Mexican War as wrong-headed bullying from the outset, and took some political heat for it: http://dig.lib.niu.edu/mexicanwar/lesson6-packet1.html