Exactly SiGen, the typical American thinks that Mexico is an extremely poor 3rd world country, while it is not. I've been to Tijuana several times, and well, it may be really dirty and the water gives you the craps, the rest of Mexico is better (it is, isn't it:? ?).
Alaska was bought from Russia, no war for that one.
During the War of 1812, the US wanted to conquer Canada, but never did. The whole Manifest Destiny says that the US has the God given right to own everything in the western hemisphere...
[edit]Note this was supposed to be a response to SiGen.