Complex sentence: However, Winterbotham's claim has been rejected by other "Ultra" participants and historians who argue that while Churchill was indeed aware that a major bombing raid would take place, no one knew what the target would be.
Split into simpler sentences: However, Winterbotham's claim has been rejected by other "Ultra" participants and by historians. They state that while Churchill was indeed aware that a major bombing raid would take place, no one knew what the target would be.