the U.S. Navy and the Imperial Japanese Navy. The Americans routed the Japanese and took control of the seas in this area. A concerted Allied land offensive between November 1943 and April 1944 was needed to occupy and hold the part of the island along the western shore in an area called "Torokina". The Americans set about establishing a wide defensive perimeter, draining swamps, and building multiple airfields for defence. Their next goal was to attack the Japanese on New Britain Island. The Marines were replaced by US Army troops. The Japanese infiltrated the mountains and jungles of Bougainville, and launched a counteroffensive against the Americans in 1944. The critical focus of their attack was at a place called "Hellsapoppin Ridge" by the Americans. In repulsing this attack, the American soldiers and airmen broke the back of the Japanese Army on Bougainville. The survivors retreated to their bases on northern and southern Bougainville, and the Americans left them to "wither on the vine" for the remainder of the war. During the 1943–45 period, more than 17,500 Japanese soldiers were either killed in combat, died of disease, or died of malnutrition. In April 1943 Admiral Isoroku Yamamoto, commander of the Combined Fleet of the Imperial Japanese Navy, was killed on Bougainville Island when his transport aircraft was shot down by American fighter aircraft during Operation Vengeance. In 1945, the Australian Army took over occupation from the Americans. Australia resumed control of Bougainville and Papua New Guinea, which became a United Nations trusteeship. The remaining Japanese on Bougainville refused to surrender, but held out until the surrender of the Japanese Empire on 2 September 1945. They were commanded by the Emperor to surrender to the Allied Australians, Americans, and New Zealanders, and they were repatriated to Japan. Beginnings of the independence movement Bougainville is rich in copper, and possibly gold. The mining of copper has been the subject of considerable social tensions over the last fifty years. Local people made two attempts at secession in protest of the mining exploitation. The Hahalis Welfare Society, established in 1959 and led by John Teosin and Francis Hagai, mounted a campaign against Australian authorities' attempts to impose a head tax. In 1962 over 60 people were injured in a series of conflicts with territorial police. By 1963 the society claimed 3,500 members spread over eight villages. In 1964, Australian business began the first attempt to