Complex sentence: They were opposed to the Montanards, who were advocating for a more radical approach to the revolution, which included the extreme idea that the only way the revolution would survive invasion and civil war was through terrorizing and executing those opposed to it.
Split into simpler sentences: They were opposed to the Montanards, who were advocating for a more radical approach to the revolution. Within this radical ideology was the idea that the only way the revolution would succeed is by the execution of those opposed to it.