Basically the title. This wouldn't happen all at once, but perhaps the US takes more in the Mexican Cession, then takes more in the Gasden purchase? At this point, I can't really think of a way for the US to annex all of Mexico except for maybe some shenanigans happened with the Zimmerman Telegram, and Mexico sided with the Central Powers giving the US justification for war? It seems a bit far fetched, but that's why I'm wondering about your thoughts on it.