Complex sentence: In 1951, Rabbi Menachem Mendel Schneerson became the seventh Rebbe of Chabad-Lubavitch, and he transformed it from a small Hasidic movement into the largest and most widespread Jewish movement in the world today.
Split into simpler sentences: In 1951, Rabbi Menachem Mendel Schneerson became the seventh Chabad Rebbe. The seventh Rebbe transformed the movement into one of the largest and most widespread Jewish movements in the world today.