Complex sentence: In 1930, Dieterle emigrated to the United States when he was offered a job in Hollywood to make German versions of American films; he became a citizen of the United States in 1937.
Split into simpler sentences: In 1930, Dieterle emigrated to the United States when he was offered a job in Hollywood to make German versions of American films. Dieterle became a citizen of the United States in 1937.