Complex sentence: Gaines'relationship with Donenfeld and National waxed and waned over the years; by the early 1940's the All-American titles were branded separately and no longer featured National-owned characters.
Split into simpler sentences: Gaines' relationship with Donenfeld and National waxed and waned over the years. By the early 1940s, the All-American titles were branded separately and no longer featured National-owned characters.