Complex sentence: I Battalion, RIR 122 relieved I Battalion, "Lehr" Regiment, which had suffered in Contalmaison from the constant British artillery-fire, its troops had lain in mud, among the sick, wounded and dead.
Split into simpler sentences: I Battalion, RIR 122 relieved I Battalion, "Lehr" Regiment, which had suffered in Contalmaison from the constant British artillery-fire. Troops had lain in mud, among the sick, wounded and dead.