The 'Battle of the Green Islands' was fought Allied and Japanese forces for the Green islands group between the islands of Bougainville and New Ireland, and was undertaken after the US landings to secure lodgements on New Britain and Bougainville (15/20 February 1944).

The primary focus of this 'Squarepeg' operation, in which the main land force element was provided by Major General H. E. Barrowclough’s New Zealand 3^rd Division, was the capture of Nissan island, which was secured after only a short land campaign. At only very limited cost in terms of casualties, the Allied operation resulted in the capture of several small atolls in the island chain, which were subsequently used to support air and naval operations focused on reducing the main Japanese base on Rabaul on New Britain.

The Green islands group, which comprises a number of small coral atolls, is located between Bougainville and New Ireland, and about 125 miles (200 km) from Rabaul, which was the location of the main Japanese base in the area. At the time of the battle, the islands were part of the Australian Territory of New Guinea.

In the wake of 'Backhander' landing on Cape Gloucester and 'Cherryblossom' landing round Cape Torokina, on New Britain and Bougainville respectively, the Allies sought to continue their 'Cartwheel' offensive operations in the region as part of the advance toward the Japanese base around Rabaul, before launching their 'Brewer' campaign for the Admiralty islands group. The Green islands group was subsequently identified as being close enough to be covered by fighters operating from Torokina, while offering the prospect of projecting Allied naval and air power closer toward Rabaul.

After the success of the Bougainville and Cape Gloucester landings, the majority of the naval forces under Halsey’s command was withdrawn to participate in the Central Pacific offensive. As a result, Halsey would not have sufficient naval support to land at Kavieng on New Ireland until April 1944. Concerned by the possible consequences of taking the pressure off the Japanese for three months, Halsey decided to take the easier objective of Nissan island, which would maintain the pressure on Rabaul and would bring Allied air power within range of the Japanese base area at Truk in the Caroline islands group.

A reconnaissance by four PT-boats on 10/11 January revealed that the passes into the lagoon were 17 ft (5.2 m) deep, sufficient for LSTs. A larger reconnaissance on 30/31 January by 300 men of the New Zealand 30^th Battalion, accompanied by US Navy specialists, was landed from the US destroyers Fullam, Guest and Hudson. In an undertaking that lasted a mere 24 hours, the reconnaissance showed that the lagoon was free of coral heads, that there were excellent landing beaches and a suitable airfield site on the eastern of the lagoon, and that there were only about 100 Japanese defenders (the precise figure was 72). The indigenous Melanesian population was hostile to the Japanese and so friendly to the Allies that the Allied command decided to omit any preliminary bombardment for the actual landings.

Nissan island is 9 miles (14 km) long and 5 miles (8 km) wide, and encloses a central lagoon. This latter can be accessed by two passes to the north-west. Nissan island is fringed by reefs to the north and south, but there are suitable landing beaches on its eastern shore. The island is heavily vegetated, and while mostly flat, it has areas of higher ground in the east and south. The population early in 1944 was about 1,147 Melanesians engaged in production of copra and taro.

During its time ashore, the reconnaissance party made contact with members of the native population, who provided intelligence, and several patrols were sent out to survey the ground and determine the dispositions of the local garrison. During this time, there were several minor skirmishes which resulted in four killed and five wounded, before the force was withdrawn by sea to Vella Lavella island.

Because the proper resupply echelons had not yet been organised, the recommendation of the reconnaissance team’s commander that his force seize the island at once was declined.

The main landings took place on 15 February after staging out of Vella Lavella and the Treasury islands group, and were commanded by Rear Admiral Theodore S. Wilkinson, whose naval force comprised 17 destroyers, eight destroyer transports, 12 LCIs, seven LSTs, and a number of smaller craft. The landing force consisted of most of Barraclough’s New Zealand 3^rd Division, 'Seabee' and 'Acorn' naval and army construction and engineer elements, and a number of other base units.

The Allied forces landed on several islands, including Nissan island, and recaptured them from the totally outnumbered Japanese forces, estimated at between 120 and 150 men. The main ground combat elements came from the three infantry battalions of Brigadier L. Potter’s New Zealand 14^th Brigade of the New Zealand 3^rd Division. The Japanese garrison comprised 12 naval watchkeepers and about 80 army personnel from Lieutenant General Hitoshi Imamura’s 8^th Area Army. This initial force had been delivered by submarine early in February, and had later been reinforced to a strength of about 120 men.

US Navy LSTs landed the Valentine infantry tanks of the New Zealand 3^rd Division’s Special Army Tank Squadron. Logistical support was provided by the 'Seabees' of three US naval construction battalions (the 33^rd, 37^th and 93^rd), as well as various other naval base and survey units. A US coastal artillery battalion was also landed to provide anti-aircraft defence, and other unit landed at this time were engineer, artillery, medical and other divisional logistic support units from the New Zealand 3^rd Division.

Before the landing, the assault force had been sighted as it steamed toward the islands, and a group of 32 Japanese aircraft was despatched from Rabaul to attack the naval task force. The landing craft and transports escaped damage, but the light cruiser St Louis, which formed part of the southern covering force of Rear Admiral Walden L. Ainsworth, was hit and suffered 23 men killed and 28 wounded. Later, a group of 12 Aichi D3A 'Val' single-engined dive-bombers appeared over the landing craft as they were forming up to the west of the islands, but the Japanese aircraft were promptly savaged by the escorting fighters of the AirSols command operating from Cape Torokina, which quickly gained control of the air over Nissan island. In all, the Japanese lost 12 aircraft. Apart from a near miss on LST-446, the landing proceeded quickly and smoothly, and after being crossloaded from the transport vessels, the infantry were ferried ashore in LCIs and LCVPs, which entered the lagoon to the south of Barahun island and disgorged their troops at several landing beaches around the Pokonian and Tangalan Plantations. In total, 5,800 personnel were landed on 15 February, of whom more than 4,200 were New Zealanders.

As the beach-head was established, there was only a brief resistance from several Japanese barges around Sirot island before a perimeter was established. As a result of concerns about the possibility of counterattack from Rabaul, elaborate defensive measures were implemented. Patrols were sent out, and carrying parties began moving stores off the beaches to points farther inland. A brigade headquarters was established around Tangalan Plantation, while Barrowclough’s divisional headquarters was set up around Pokonian Plantation. The perimeter was held throughout the night. On 16 February, the New Zealand infantrymen began to push inland across the island. The 30^th Battalion cleared the eastern side of the island around the Pokonian Plantation, while the 37^th Battalion cleared the northern area of the island from Tangalan Plantation, the 35^th Battalion pushed to the south. Each battalion was supported by a troop of Valentine tanks, which provided close support and helped clear tracks through the jungle.

A series of minor patrol actions took place in the 30^th Battalion’s area of responsibility, while a group of around 70 Japanese was encountered around a Roman Catholic mission around the southern end of the island near Tanaheran, having become caught between patrols from both the 30^th and 35^th Battalions. This group of Japanese were overwhelmed over the course of several days by New Zealand infantry, supported by several tanks, at the cost of three men killed and 11 wounded, while 62 Japanese were killed. Further ground fighting took place on 19 February, before the island was finally declared secure on the following day. On 23 February, the final action of the campaign took place when a company-sized patrol of the 37^th Battalion cleared the small island of Sau, to which 14 Japanese survivors had withdrawn. After refusing a call to surrender, the small Japanese force was destroyed in a brief firefight that resulted in four New Zealanders being wounded.

Overall Allied casualties for the entire operation amounted 13 men killed and 26 wounded, while almost all of the Japanese garrison was killed.

The capture of the Green islands group further isolated Rabaul and Kavieng, and completely isolated the Japanese on Bougainville from any outside assistance.

Base development on Green island was the task of the 'Seabees' of the 22^nd Naval Construction Regiment. This was activated on 15 January 1944, and comprised the 15^th, 3^rd, 37^th and 93^rd Naval Construction Battalions. At the time, the 15^th and 93^rd Battalions were in the Russell islands group, the 33^rd Battalion was en route there from New Zealand, and the 37^th Battalion was at Ondonga airfield on New Georgia island. Units of the 22^nd Naval Construction Regiment began moving to the Green islands group on 15 February, and work began on the construction of an airstrip on 20 February. By 6 March, a 5,000-ft (1525-m) by 150-ft (46-m) airstrip had become operational, and US Marine Corps' fighters of the VMF-22 and VMF-223 squadrons based there attacked targets in Kavieng and other parts of New Ireland. Work continued on roads, taxiways, hardstands and base facilities, and a 6,000-ft (1830-m) by 150-ft (46-m) bomber strip had been completed by the end of the month. The work was hampered by bad weather, the need to clear away large trees and dense underbrush, and the presence of rocks that required blasting. Coral used for the surfacing was obtained from distant quarries and then had to be transported to the airstrips. The air base was supported by a fuel farm with 14 tanks.

The airstrips became home to US Marine Corps' fighters and bombers which participated in the Allied effort to isolate the Japanese bases on Rabaul and Kavieng, while New Zealand fighters also used the base to refuel during operations over Rabaul. A South Pacific Combat Air Transport Command detachment, which supplied matériel and mail to combat soldiers and evacuated the wounded, was also established on the island. Other base facilities included a 450-ft (137-m) by 250-ft (76-m) coral seaplane ramp, and three moorings with concrete anchors and oil drum buoys. A fuel pier was constructed, and an entire PT-boat base with camps, workshops, a steel warehouse and a T-shaped pontoon pier. Medical facilities were provided for a naval base hospital with four Quonset huts. As a result of food shortages on the island as it was built as a base by the Allies, the entire native population was transferred to Guadalcanal island in March 1944.

When airstrips on Emirau island, in the St Matthias islands group to the north-west of Kavieng, were opened in May 1944 after the islands' seizure on 'Beefsteak', the importance of Nissan island declined as aircraft were transferred there. Base construction was considered complete by July 1944, and responsibility for the installations was handed over to the 552^nd and 553^rd Construction Battalion Maintenance Units. These began to dismantle the base late in 1944. The 552^nd CBMU left in March 1945, but the 553^rd CBMU remained until August 1945.