Complex sentence: In September 1940, he founded a breakaway militant Zionist group named Lehi, called the "Stern Gang" by the British colonial authorities and by their assistants in the Yishuv establishment.
Split into simpler sentences: In September 1940, he founded a breakaway militant Zionist group named Lehi, also known as the Stern Gang. It was called the "Stern Gang" by the British colonial authorities and by their assistants in the Yishuv establishment.