Complex sentence: Even though it was Erich von Manstein who had developed the "sickle cut" strategy, he was not promoted to field marshal by Hitler until after victories on the Eastern front in 1942.
Split into simpler sentences: Even though it was Erich von Manstein who had developed the "sickle cut" strategy, he was not promoted to field marshal. Hitler would instead promote Manstein to field marshal after victories on the Eastern front in 1942.