Complex sentence: The movement claimed that was the first amongst the Lebanese "progressive" militias during the war, and by 1977 it was the largest organization within the LNM-JF, both in terms of popular support and military capacity.
Split into simpler sentences: The movement claimed was the first of the Lebanese "progressive" militias during the war. As of 1977 it was the largest organization within the LNM, both in terms of popular support and military capacity.