In the Spring of 1944, U.S. and Australian forces began "island hopping" to the Philippines, and the airfields on the island of Biak were the first U.S. objective. Planners forecast a week-long operation, unaware that there were 11,500 Japanese on the island entrenched in large caves which honeycombed Biak's mountainous interior. On May 27, 1944, the experienced veterans of the 41st Infantry Division, made up originally of National Guardsmen from Oregon, Washington, Idaho and Montana, landed unopposed on Biak. It was a feint: the 41st, which had earned its "Jungleer" nickname in New Guinea, was about to enter its toughest fight of World War II. Oregon's 162nd and 186th Infantry were moving inland toward the airfields when the enemy struck. The 163rd Infantry (Montana) was quickly brought in to join the horrible jungle fighting, where despite the heat and steep terrain, water was rationed to one canteen every 24 hours. On June 15, I Corps commander Lt. Gen. Robert Eichelberger assumed command, prompting a new plan of attack. On June 16, the 2nd Battalion, 186th Infantry, attacked to close a gap between their regiment and the 162nd. The battalion overran dozens of machinegun nests, log bunkers and even a naval gun, and opened the battle's second phase when they discovered a second major cave system. These caves were not cleared until June 27, and not until August 20 would the fight for Biak be declared over. The brilliant Japanese use of natural defenses foreshadowed later fighting on Pelileu and Okinawa, and as on those more famous islands, the fighting on Biak was costly. Only 10 percent of the Japanese defenders survived; the U.S. suffered some 400 killed and 2,000 wounded in action, most of them from the 41st Division. Today, the proud heritage of the Jungleers is carried on by the 162nd and 186th Infantry Regiments of Oregon's 41st Infantry Brigade, and by Montana's 163rd Infantry and 163rd Cavalry Regiments.