Complex sentence: Soviet war casualties accounted for the highest proportion of the conflict, in the cost to acquire the upper hand over Axis forces at intense battles such as Stalingrad, eventually driving through Eastern Europe and capturing Berlin in 1945, inflicting the vast majority of German losses.
Split into simpler sentences: Soviet war casualties accounted for the highest proportion of the conflict in the cost of acquiring the upper hand over Axis forces at intense battles such as Stalingrad. Soviet forces eventually drove through Eastern Europe and captured Berlin in 1945, inflicting the vast majority of German losses.