Complex sentence: The German Air Force (Luftwaffe) played a vital part in the siege, bombing the besieged Soviet forces with impunity and dropping 20,528 tons of bombs on them in June alone.
Split into simpler sentences: The German Air Force (Luftwaffe) played a vital part in the siege. It provided highly effective aerial bombardment in support of the ground forces by dropping 20,528 tons of bombs on the besieged in June alone.