He became one of the leaders of the Jewish defense effort in Mandate Palestine and chief architect of the Haganah, the underground military organization for defense of the Yishuv between 1920 and 1948.
He played a key role as a leader of the Jewish defense coalition in Mandate Palestine and served as the primary architect of the Haganah, the covert military group protecting the Yishuv from 1920 to 1948.