Philbrick was an American pacifist who joined the communist party that opposed WWII, until Barbarosa. When the party changed its opposition he realized they were Stalinistas and Went to work for the FBI.
I don’t think he was a naif turned off by Barbarossa. He had joined the party to protest the war, but contacted the FBI when he began to suspect their true agenda. He had been an FBI informant for nearly two years before Germany invaded Russia.