Complex sentence: Watson soon began second-guessing himself for accepting the medal when any involvement of Americans with Germany was frowned upon and as war between the USA and Germany looked imminent, and eventually returned the medal to the German government in June 1940.
Split into simpler sentences: Watson soon began second-guessing himself for accepting the medal, when any involvement of Americans with Germany was frowned upon and as war between the USA and Germany looked imminent. He returned the medal to the German government in June 1940.