Complex sentence: Despite horrific casualties, the rebels seized two-thirds of the town, using the smoke from blazing fires for cover, and forced the near withdrawal of all Crown forces but a final counterattack in the afternoon finally drove out the exhausted rebels.
Split into simpler sentences: Despite horrific casualties the rebels managed to seize two-thirds of the town by using the cover of smoke from the blazing town and forced the near withdrawal of all Crown forces from the town. However the military managed to hold on and a final counterattack in the afternoon finally drove out the exhausted rebels.