As a wit far greater than mine so eloquently put it in an old MWAN, it is the war gamer's loss that with 4 Corps of the Army of the Potomac, with Grant, Sherman, and Sheridan on the Rio Grande, and Mexico full of Republicans, Juaristas, French (including Foreign Legion), Austrians, Belgians, Sudanese, and others, and the whole powder keg ready to blow, that tragically "Peace broke out."
Nappy III had done his level, if passive aggressive, best to undermine and ultimately destroy the United States in his crusade to save the world from Democracy, but once the US had both hands--and tens of thousands more--free to reassert the Monroe Doctrine, he pulled the plug on his schemes, and left Maximilian alone to pay the Mariachi's.
I'm not sure a more stunning example of poltroonery and betrayal exists in the historical record, at least of the 19th Century.
Nappy III was--or was certain he was--too clever to support the South directly during the war, but put enormous resources into Mexico in order to establish his beach head and counterweight to the US. But if the Vietnam War seemed destined to last forever (almost 30 years anyway), trying to subdue Mexico would have taken at least as long, if not longer, and the French and their "allies" never really came to understand that until the US was suddenly able to get fully into the game. An atrocity filled war in Mexico could have gone on indefinitely, but if Nappy was looking for an excuse for not being able either to kill or persuade enough Mexicans to love him and his (far more honorable) puppet, he finally found it.
The chogie the French and Allies cut pulling out of Mexico would have looked like a rout if Nappy had not pointedly made sure to take all their artillery with him, leaving poor Maxel with little more than his last-century museum pieces and an "Imperial Army" better dressed for Mardi Gras than serious fighting.
Though additional thousands would have died, it still seems at this remove a major let-down that the US didn't get its chance to send the then most experienced modern army in the world over the Rio Grande to give the peon hangers a genuine all American ass whuppin'.
Maybe Wilhelm and Bismarck would have been spared their trouble of a few years later, to the benefit of both France and Germany. And who knows what benefits Mexico might have received from a US actively supporting Benito Juarez in establishing a just and democratic government?
Ah, the "What If's" of history….
TVAG