Much like Cosa Nostra in Sicily, the Corsican mob was very active in the Resistance against the nazi and in helping for the D-Day.

As a reward, after the war, the French authorities granted to the Corsicans the exclusive rights to run gambling club in Paris (kind of like the indians and the Casinos in the US). There were very few gambling clubs in Paris and all run by the Corsicans, until very recently (they all close because they were linked to Corsican organized crime...).

The Corsican mob was able to flourish because it had no competition in France. They were by far the most organized and dangerous criminal groups in the country so they could freely take over the rackets in Marseille and Paris.