Complex sentence: December, 1942, codenamed Operation Mars, was nearly as bloody as the first, and also failed, although the Soviets tied down German forces which may have otherwise been transferred to the Battle of Stalingrad which was then raging.
Split into simpler sentences: December, 1942, codenamed Operation Mars, was nearly as bloody as the first, and also failed. Although the Soviets tied down German forces which may have otherwise been transferred to the Battle of Stalingrad which was then raging.