Germany was trying to get Mexico to join them for wwi, with enticement of getting back the land they lost 70 years prior. That loss was immense for Mexico in land, pride, and economics

Mexico was dealing with its own internal issues (revolution) which made it difficult for them

A slight turn of events and the us would have a huge southern front I deal with, and a base of support for disaffected native Americans and African Americans.

Russia also dropped out of Wwi due to its intern revolution

It is easy to look back and see manifest destiny as a given. There were a lot of contingencies