Complex sentence: Upon completing his training on March 31,1943, Glenn was commissioned as a second lieutenant and, after advanced training at Camp Kearny, California, was assigned to Marine Squadron VMJ-353, flying R4D transport planes.
Split into simpler sentences: Upon completing his training in March 1943, Glenn was commissioned as a second lieutenant. After advanced training at Camp Kearny, California, he was assigned to Marine Squadron VMJ-353, flying R4D transport planes.