The battle of Saidor (2 January 1944) saw US troops land between the remaining Japanese bases on the north coast of the Huon Peninsula. As a result the Japanese abandoned all of their bases to the east of the landings.
Australian troops had been fighting to gain control of the Huon Peninsula since 22 September 1943, when they landed north of Finschhafen. That port fell in early October, and the Australians then repulsed a determined Japanese counterattack. After that they went onto the offensive, clearing the Japanese out of Sattelberg and Wareo in the area to the north-west of Finschhafen. After that they began to advance north along the coast towards the Japanese held port of Sio.
While the Australians were fighting on New Guinea, the Americans were preparing to land on New Britain, on the opposite side of the Vitiaz Strait from the Huon Peninsula. The plan was to land at Arawe on the southern side of the island on 15 December 1943 to distract the Japanese, then at the more important Cape Gloucester, on the north-western corner of the island, on 26 January 1944. During the planning for this operation the benefits of a landing on the north coast of the Huon Peninsula began to appeal to the Americans. This would take advantage of the US naval presence in the area and hopefully speed up the end of the fighting on the Huon, making the naval links to Cape Gloucester more secure.
Detailed planning for the attack began in mid December. On 17 December General Krueger, commander of the US Sixth Army, was ordered to prepare to take Saidor on or before 2 January 1944, as long as the fighting around Cape Gloucester on New Britain was going well. The attack was to be carried out by the 126th Regiment of the 32nd Division, commanded by Brigadier-General Clarence A. Martin. Unfortunately Martin's orders were rather limited. He was to capture a large enough area to allow for uninterrupted operation of air and naval facilities at Saidor, and to help in the creating of a naval base and airfields.
The Japanese had a harbour and an airfield at Saidor, but very few troops.
The attack began with a bombardment by units of the 7th Fleet. Poor weather delaying the landings until 7.25am, but there was very little opposition and by the end of 2 January 8,000 US troops were ashore. On the following day the Americans established a perimeter and began to send out patrols along the coast and inland.
On 4 January General Adachi arrived at the now isolated Japanese base of Sio (travelling by submarine), and decided to abandon the remaining positions on the Huon Peninsula. The 20th and 51st Divisions were to retreat west to Madang, travelling inland to avoid any clashes with the Americans at Saidor. The retreat began on 6 January. This was the start of a devastating march through the Finisterre mountains which cost the two divisions half of their men.
The Americans at Saidor could have made the Japanese retreat much harder. The 128th Regiment arrived in the beachhead on 19 January, and Martin was aware that he had enough men to push east and interrupt the retreat. Unfortunately General Kreuger wasn't yet ready to go onto the offensive. He believed that there was still a danger of Japanese counterattacks around Saidor. Wet weather also played a part.
Despite the relative inactivity of the Saidor garrison, the landings did have a major impact on the New Guinea campaign. As well as abandoning Sio, General Adachi decided to withdraw from the western Finisterre Mountains. He ordered his men to abandon their last positions on Shaggy Ridge, and to only hold the nearby Kankiryo Saddle for long enough to allow the 21st and 50th Divisions to escape.
Advancing Australians from the 5th Division reached Saidor on 10 February 1944. The Allies then moved west, soon joining up with forces coming over the Finisterre Mountains. Further afield the Allies invaded the Admiralty Islands on 29 February 1944. General Adachi, the Japanese commander in eastern New Guinea, decided to abandon his major base at Madang, and Allies made an unopposed entry into the town on 24 April.