Complex sentence: The Marines by that time had received reinforcements, both personnel and some newer and bigger guns, all of which were used by the Marine garrison when they engaged attacking Japanese A6M2 Zeros in June 1942.
Split into simpler sentences: The marines by that time had received reinforcements, both personnel and some newer and bigger guns. All of which were used by the marine garrison when they engaged attacking Japanese A6M2 Zeros in June 1942.