Complex sentence: He joined the U.S. Marine Corps in 1942 and served with I Company, 3rd Battalion, 23rd Marines of the 4th Marine Division through the battles of Roi-Namur in the Kwajalein Atoll, at Saipan and at Tinian.
Split into simpler sentences: He joined the U.S. Marine Corps in 1942 with I Company, 3rd Battalion, 23rd Marines of the 4th Marine Division. He served with the 23rd Marines through the battles of Roi-Namur in the Kwajalein Atoll, at Saipan and at Tinian.