Complex sentence: Their orators had no serious rivals in the hostile camp--their system was established in the purest reason, but the Montagnards made up for what they lacked in talent or in numbers through their boldness and fanatical energy.
Split into simpler sentences: Their orators had no serious rivals in the hostile camp; their system was established in the purest reason. But the Montagnards made up for what they lacked in talent or in numbers through their boldness and fanatical energy.