Complex sentence: During the Second World War, the Kerch Peninsula was the scene of much desperate fighting between forces of the Soviet Red Army and Germany, which intensified in the coldest months of the war when the straits froze over, allowing the movement of troops over the ice.
Split into simpler sentences: During the Second World War, the Kerch Peninsula was the scene of much desperate combat between forces of the Soviet Red Army and Germany. Fighting frequently intensified in the coldest months of the war when the straits froze over, allowing the movement of troops over the ice.