Complex sentence: Although Parrish's proposal was defeated, Miller left the union as executive director, but sources differ whether he was forced out or quit, or forced out because he was not hawkish enough.
Split into simpler sentences: Although Parrish's proposal was defeated, Miller left the union as executive director. It is clear that he left the union, but some sources say he quit, while others say he was forced out because he was not hawkish enough.