When V Amphibious Corps were preparing for the invasion of the Marianas Islands--Saipan, Guam, and Tinian--they were expecting a relatively easy fight. The Japanese appeared to be on the run. As D day for Saipan (the first of the three islands scheduled for conquest) loomed, V Corps planners felt safe in allocating a single army division as corps reserve for the conquest.   As Lt. Col. William J. O'Brien's First Battalion and the 105th Infantry landed on Saipan, they had little idea what was in store for them. Enemy opposition was fierce. For the next several weeks they faced the unremitting terror of nearly continuous combat. For the 105th Infantry, the battle climaxed in an overwhelming Japanese banzai attack July 7, 1944. The regiment suffered more than 900 casualties, almost half of whom were killed in action, including First Battalion's commander, William O'Brien, who later received the Medal of Honor for his efforts. Throughout the battle, O'Brien provided a stirring example of frontline leadership to his previously untested troops. His story is just as inspiring today.