The Battle of the Green Islands or Operation Squarepeg was fought from 15 to 20 February 1944, between Imperial Japan and Allied forces from the New Zealand 3rd Division and the United States.

[5] The atolls form a rough circle, with the main island, Nissan, forming a west-facing crescent shape closed off by several smaller islands separated from the main landmass by small passages that open up into a large internal lagoon roughly 4 nautical miles (7.4 km; 4.6 mi) in width.

The Green Islands were subsequently identified as being close enough to fighter cover provided by aircraft based at Torokina, while offering the prospect of projecting Allied air and naval power further towards Rabaul.

[5] The reconnaissance party remained ashore for 24 hours, during which contact was established with 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this time, there were several minor skirmishes which resulted in four killed and five wounded, before the force was withdrawn by sea to Vella Lavella.

[8] Under the command of Admiral William Halsey Jr.,[9] the invasion of the islands commenced on 15 February 1944, codenamed "Operation Squarepeg".

The landing craft and transports escaped damage, but the cruiser St. Louis, which formed part of the southern covering force, was hit resulting in 23 killed and 28 wounded.

[19] After being crossloaded from the transports, the infantry were ferried ashore in LCIs and LCP(R)s, which proceeded to enter the lagoon to the south of Barahun Island.

[24] On 23 February, the final action of the campaign took place when a company-sized patrol from the 37th Battalion cleared the small island of Sau, where 14 Japanese survivors had withdrawn.

Work continued on roads, taxiways, hardstands and base facilities, and a 6,000-by-150-foot (1,829 by 46 m) bomber strip was completed by the end of the month.

The work was hampered by bad weather, the need to clear away large trees and dense foliage, and the presence of rocks that required blasting.

Lieutenant Richard Nixon, who later became President of the United States, served as the officer-in-charge of the SCAT detachment on the Green Islands in early 1944.

[29] Other base facilities included a 450-by-250-foot (137 by 76 m) coral seaplane ramp, and three moorings with concrete anchors and oil drum buoys.