Complex sentence: Soon after the German invasion, in August 1941, he was appointed as deputy chief of operational planning in the General Staff, rapidly rising through the ranks in the Operations Department until becoming it's chief in May 1943.
Split into simpler sentences: Soon after the German invasion, in August 1941, he was appointed as deputy chief of operational planning in the General Staff. He rapidly rose through the ranks in the Operations Department until he became it's chief in May 1943.