Operation Goodtime - the invasion of the Treasury Islands (27 October 12 November 1943) - was a preliminary operation before the main invasion of Bougainville Island in the Solomon Islands. The American aim was to seize the islands and build airfields that could be used to support the fighting on Bougainville, where the Japanese had a strong garrison.
The attack on the Treasury Islands was to be carried out by the southern force of Task Force 31, under Admiral Fort. He was given eight destroyer transports, two LSTs, eight LCIs, 4 LCTs and two APCs. The invasion was to be carried out by the 8th Brigade Group of the 3rd New Zealand Division, supported by the 198th Coast Artillery, A Company of the 87th Naval Construction Battalion and a number of support units.
There are two main islands in the Treasury Island group. Mono Island in the north is the largest, and is almost rectangular. Stirling Island, to the south, is much smaller and irregular in shape, although it would prove to be large enough to contain a runway. The strait between the two islands is called Blanche Harbour. Stirling Island was undefended, but the Japanese had around 225 men from the Special Naval Landing Forces on Mono Island.
Admiral Fort split the force heading for the Treasury Islands into five transport groups, all of which were intended to reach Blanche Harbour between 5.20 and 8.30am on 27 October. He hoped to reach Blanche Harbour undetected, but early on 27 October a Japanese reconnaissance aircraft spotted one of the transport groups and the escort group. The landing force wasn't attacked before it reached the Treasuries, but the Japanese were warned that it was coming.
The landing took place as planned. Stirling Island was occupied during the morning without any fighting. The troops landing on Mono Island faced some opposition on the beach, but this was easily overcome. A more serious threat came from hidden mortar positions that opened fire at the LSTs of the fourth wave. By noon the New Zealanders had pushed out from the landing beaches, and eliminated the mortar positions. The remaining Japanese troops retreated into the hilly interior of the island. A second force landed at Soanotalu, on the northern coast of Mono, but found no Japanese.
The only serious threat to the landing force was provided by aircraft based on Rabaul. These arrived after the Allied ships had unloaded and attacked them during their return journey to Guadalcanal. The destroyer USS Cony was damaged but all of the ships returned safely to base.
The fighting on Mono Island lasted for two weeks. The Japanese launched a series of small counterattacks on the night of 27-28 October, but after that was largely left alone. The New Zealanders then began a period of sweeps through the jungle, which lasted until 5 November. Small groups of Japanese stragglers were found and destroyed during these operations.
The Soanotalu beach head wasn't attacked until 29 October, and then only by small parties of Japanese troops attempting to retreat from the south coast. The largest attack came on the night of 1-2 November when a force of around 80-90 Japanese troops made an attempt to break through to the beach and steel a landing craft. They were repulsed with heavy losses. A similar, but smaller, attack was repulsed on the following night,
By 12 November the last Japanese troops had been killed or captured. The New Zealanders lost 40 dead and 145 wounded, the Americans 12 dead and 29 wounded during the two week battle. The Japanese lost at least 205 dead.
American engineers quickly got to work building a new base on the Treasury Islands. A naval base and radar were soon in place, and a 5,600 foot long runway was ready by Christmas Day, in time to play a part in the later stages of the fighting on Bougainville.