I read somewhere that the US was proposing to purchase land from north Mexico during the American/Mexican war in 1845, but never went through. The proposed border was the south border of Sonora, Chihuahua, Coachuila, Nueva Leon and Tamauupas. What do you guys know about the history of this event?
It was the war, as you say, but, of course, that was in 1847/8. When Mexico was properly defeated, Polk thought he'd get a better deal than the initial proposal that Mexicans had already, essentially, agreed to. It wouldn't have been a purchase of land, in the sense that no Mexican government would have agreed to even talk about it, if not for the fact that Mexico City (and much of the rest of the country) had been occupied. Anyway, the US North was dead set against it: annexation of a bunch of extra Texases, all of which would have likely become slave states, would have destroyed the ballance between North and South. If I recall correctly, some Northern politicians countered with a proposal to annex all of Mexico - which everyone knew to be completely impossible. That is my vague recollection - you'd do better to check the books for details.
I read quite a bit on this subject a couple of years ago. Polk wanted California and other parts of northern Mexico and was willing to pay for them. The Mexicans wouldn't consider such a deal, so Polk provoked a war. There was considerable political difference about annexation. Most Whigs were opposed to any. Most Democrats wanted some, including a few who wanted all Mexico. Nicholas Trist, who negotiated for the US, had already been relieved of his post, but went ahead negotiating anyway. Polk wasn't thrilled with the treaty, but decided to accept it
