The battle of Nassau Bay (30 June 1943) was an early step in the wider Allied offensive in the Huon Gulf area of New Guinea (Operation Postern), and was carried out in order to capture a staging post for later steps in the campaign and to improve the supply situation for the main Australian force attacking Salamaua from inland bases.
The main Allied objective in the southern Pacific in 1943 was to isolate the Japanese base at Rabaul. The steps to be taken to achieve this were laid out in the Elkton III plan, and implemented under the overall codename Operation Cartwheel. The attack on the Japanese bases at Salamaua and Lae in the Huon Gulf was the first stage in Operation Postern (Operation II in the Elkton III plan). The first stage in the plan, Operation Chronicle, was the occupation of the undefended Woodlark and Kiriwina Islands.
The landings in Nassau Bay were carried out to support Operation Postern, but they took place at the same time as Operation Chronicle. Lae was to be attacked by an amphibious force, and this would need a staging post within landing craft range. Nassau Bay was outside the main Japanese defensive perimeter south of Salamaua, and could be occupied without too much of a fight.
This location had a second advantage, in that the Americans landing by sea could quickly join up with the Australians coming from Wau. The Australians had maintained a base at Wau, across the mountains to the west of Salamaua, since the Japanese first arrived in the area, and had repulsed an attack on that base in January 1943. In February they pushed the retreating Japanese back as far as Mubo village, but the advance then came to a halt. The Australians had to be supplied by porters bringing everything across the mountains between the frontline and the airfield at Wau, and this greatly limited the amount that could be carried.
The plan was for a two pronged move to Nassau Bay. The Australians would send a small force to the area from the west to erect guiding lights on the beach, while the Americans would come from the sea. Another Australian force would move to the mouth of the Bitoi River, just north of Nassau Bay.
The Japanese were believed to have 300-400 men at Nassau Bay, and another 75 on the Bitoi River, a little further force. In fact they only had 150 men in the area, 100 from the 102nd Infantry, 51st Division and 50 from the Navy. They did have a bunker on Nassau Bay, but it played no part in the landings.
The attack was to be carried out by MacKechnie Force, which consisted of the reinforced 1st Battalion, 162nd Infantry, 41st Division, a company of Papuan soldiers, one platoon of an Antitank Company, one Engineer company, the 218th Field Artillery Battalion and various support and medical detachments. The force was commanded by Colonel Archibald R. MacKechnie. It had been formed in March, and advanced along the coast from Buna to Morobe, where it was used to guard a PT boat base.
On the night of 28-29 June the regiment's Intelligence and Reconnaissance Platoon captured a series of small islands between Morobe and Nassau Bay and installed guide lights.
The landings didn't go entirely to plan. The fleet was made up of 29 LCVPs, two Japanese barges and one LCM, split into three waves. Each was to escorted by a PT boat carrying more troops, and a fourth PT boat provided a small naval escort. The first two waves met their guide safely, but the third had to travel unescorted. The weather was poor, with heavy rain and ten to twelve foot high waves breaking in the bay. The Australian platoon arrived late but still managed to put up two guide lights. The first wave overshot and had to come back. The surf caused the biggest problems. Nineteen of the LCVPs were forced too far up the beach and wrecked. The LCM landed a bulldozer on its first trip, but was swamped on its second trip.
The Japanese did have a few men posted on the bay, but they mistook the bulldozer for a tank and fled into the jungle. The Americans got 770 men onshore during the night, from the first and second waves. The unescorted third wave arrived after the initial disasters, and its commander decided not to risk a landing while the surf was so high. In the end the troops in third wave didn't land until 23 July.
By dawn on 30 June the Allies had set up a defensive perimeter, with US troops 300 yards to the north and south and the Australians to the west. The disastrous landing had at least freed up a number of machine guns to reinforce the perimeter, but it had also knocked out most of the force's radios, and communications were poor for some time.
After dawn on 30 June the Americans began to probe to the north and south. In the north they ran into Japanese gunfire, and it needed a stronger attack during the afternoon to clear the Japanese from the area between Nassau Bay and the Bitoi River.
The Japanese commanders decided not to risk a full scale attack on the new American beachhead. The Japanese did carry out one reasonably large scale attack on the beach, but this was carried out by troops who had been cut off further to the south. In the late afternoon they clashed with the American troops who had been sent south, and forced them back to the beachhead. A new defensive line was set up at the southern edge of the bay.
That night the Japanese attacked. For many of the American troops this was their first experience of combat, but they were able to repulse the night attack. This attack cost the Americans 18 dead and 27 wounded, the largest number of casualties suffered during the landings. The Japanese lost at least 50 dead.
By 2 July, with the rest of the landing force finally on the beach, Nassau Bay was declared secure. On the same day the Americans moving west made contact with the Australian 17th Brigade around Mubo.
The combined forces then turned their attention towards the Japanese at Salamaua, advancing slowly across difficult terrain during July and August, before the Japanese finally evacuated both Salamaua and Lae in mid-September.