Well... Just say that Mexico did not have that mass immigration. Lets say they just kept what was theirs, so California, the Rockies (parts of it), and Texas, plus the rest of Mexico of course. Maybe it would be the Mexicans who would "save" Europe in the WWs? Or Mexico and USA together? Maybe Mexico and USA would become real good palls and trade and stuff, and together with Canada form a comunity "over there" that would be just as strong as it is now? Maybe stronger?
So maybe it would have been Mexicans who would fight Japanese in British and French colonies in south eastern Asia? Or help out the Brits in Africa?
Or maybe they'd side with Germany/Italy/Japan and invade USA, with a bit help from the Japanese they could perhaps make it?
I've never thought much of Polks (that was the guy right, the president?) conquests, but yeah, they might have been vital for USA...