Complex sentence: He was the secretary of the prominent Betar Warsaw organization in 1938 before leaving it to join, and quickly take leadership of, the left-wing Zionist Hashomer Hatzair group in Warsaw.
Split into simpler sentences: He was the secretary of the prominent Betar Warsaw organization in 1938. He left it to join and quickly take leadership of the left-wing Zionist Hashomer Hatzair group in Warsaw.