Complex sentence: Padfield suggests that Churchill rejected this proposal because he was hopeful that the United States would join the war, he wanted to maintain the existing alliance of exiled European governments, and he wanted to maintain his own domestic political position.
Split into simpler sentences: Padfield suggests that Churchill rejected this proposal because he was hopeful that the United States would join the war. He also wanted to maintain the existing alliance of exiled European governments and to maintain his own domestic political position.