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The petition must happen between the years 1900 and 2012. No part of your timeline can include an apocalypse or collapse of civilization. POD can be anytime after the end of the Mexican-American war, although of course the later your POD, the more impressive.

History as a whole can get as crazy as you want, but in the context of your proposal, it must be internally sensible for the majority of Mexico's government and people to approve of the move. No rules-lawyering! (e.g., having a tiny state calling itself mexico petitioning for annexation.)

Bonus round: Have the United States be in a position where it wants to accept.
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