Complex sentence: Riding on this victory, the Montagnards then sought to discredited the Girondins using tactics previously used against them, denouncing the Girondins as liars and enemies of the Revolution.
Split into simpler sentences: Riding on this victory, the Montagnards then sought to discredited the Girondins. They used tactics previously employed by the Girondins to denounce them as liars and enemies of the Revolution.