 The reduction of the Japanese pocket around Umurbrogol mountain has been called the most difficult fight that the U.S. military encountered in the entire war.The 1st Marine Division was severely mauled and it remained out of action until the invasion of Okinawa began on April 1, 1945.In total, the 1st Marine Division suffered over 6,500 casualties during their month on Peleliu, over one third of their entire division.