Complex sentence: The three battalions of RIR 212 had assembled at midnight on the eastern slope of Broodseinde Ridge and had been guided to the front line, ready to move forward and hug the barrage.
Split into simpler sentences: The three battalions of RIR 212 had assembled at midnight, on the eastern slope of Broodseinde Ridge. The troops had been guided to the front line, ready to move forward and hug the barrage.