Complex sentence: At times during the war they served in the trenches as infantry, each cavalry brigade when dismounted formed a dismounted regiment, so when the division went into the front line, they could only cover a brigade area.
Split into simpler sentences: At times during the war they served in the trenches as infantry, each cavalry brigade when dismounted formed a dismounted regiment. This meant that when the divisions went into the front line, they could only cover a brigade area.