Complex sentence: Because of its long association with Toho, UPA is better known to cult-movie fans today as Toho's American distributor rather than a pioneer of animated cartoons, but the legacy of UPA is an important chapter in the history of American animation.
Split into simpler sentences: Because of its long association with Toho, UPA is better known to cult-movie fans today as Toho's American distributor rather than a pioneer of animated cartoons. But the legacy of UPA is an important chapter in the history of American animation.