AHC: make Mexico be dragged on to American Civil war.

Maybe have the Royalists win during the French intervention? During the American Civil War the Confederates sent men and supplies to bolster the Royalists whilst the Union sent men and supplies to the Republicans. If the Royalists pull off a stunning victory they might become concerned that a victorious Union, citing the Monroe Doctrine, might invade and restore the Republic. As a result they enter on the side of the Confederates in the hope of propping up a friendly country on their border.
 
Have the French intervene in favor of the Confederacy after having already begun intervening in Mexico. At that point, the Juaristas and the Union are both fighting the French, and will want to coordinate their actions. I don't see Mexicans fighting the Confederates directly (too likely to backfire politically), but joint Union-Mexican operations against the French are certainly a possibility (especially on the naval end, with US Navy ships supporting Mexican land forces).

I don't see either Mexican side intervening after their own war is over; by this point Mexico has been in civil war since 1857, is heavily in debt, and has no appetite for foreign adventures.

Napoleon III was always more sympathetic to the Confederacy than the British were.
 
TFSmith did manage to get the Mexicans to assist the Union (not join them, mind, just some small assistance) in his timeline in exchange for retroceding the Gadsden Purchase.
 
Top