Serge Gainsbourg was of course… a fag from the Fifth Marseille, casi esquina con bis de Verneuil y Saint-Germain-des-Prés. Anyhow, David Martínez at Quora:
Why did the French not conquer Mexico?

The Newspapers say, sr. Belmont, de Paso del Norte, —not the Pancho en RFPP 106.3 fm at The Puebla Pavillon de la República Bolivariana de Guanabacoa, that:
It’s actually quite impressive Mexico survived. In the course of 30 years, it got invaded by the US and lost half its territory, Santa Anna was disgraced, a new constitution was signed, a reform war was waged, and the country was invaded by the French.
It was a collection of liberals, educated in enlightenment and French Revolution principles – Benito Juárez, Melchor Ocampo, Guillermo Prieto, Riva Palacio and others – that saved Mexico from falling to Imperialists.
And the song Mexicans would sing at night, within earshot of the French troops, whenever they’d win?
The Spanish version of La Marseillese–esa
Which, since it was the song of the French Revolution that had before, made them (filthy French) a Republic, was forbidden in France, by Napoleón Tercero de La Place des Vosges.

… The attempt on the Mexican Republic carries the attempt on the French Republic.
One ambush completes the other. The empire will fail on its infamous intention, that I believe.
And you will win. But either way wether you win or be defeated, France will continue to be your sister, sister in your glory or your misery.
And I whose name you call will tell you again that I am with you. If you shall win I offer you my fraternity as a citizen.
If you shall be defeated I offer you my fraternity as an outlaw.
Eventually, the French Empire died and the Mexican Republic lived on.