You may remember from your history books, or perhaps from this blog, that in 1861 Emperor Napoleon III invaded Mexico with hopes of making it a puppet state of the French empire. Hugo was an anti-monarchist and opposed the invasion.
He began his letter...
You are in the right, and I am with you.
You can count on my support.
And you have to know that it is not France that is making war against you,
it is the Empire.
He goes on to say in part...
Valiant men of Mexico!
Resist the perfidy and treachery. And if you do, you will overcome.
But know that in victory or defeat,
France will always be your sister.