Which regions of present - day Vietnam were particularly dangerous for the French forces?

The Iron Triangle region had been dangerous for the French.

Explanation:

The viet war of independence was one of the bloodiness and rarely successful that some eastern power had fought against the Europeans for control of their land and ended up winning.

The region that is being talked of here is called the Iron Triangle and in the time of the war it was being controlled by Viet Minh.

The gorilla forces uses the treacherous terrain of the region to their advantage and had trained to be good fighters there which is not the case for the french who lost in there repeatedly.