The 'Neutralisation of Rabaul' was the Allied campaign to render useless the Japanese strategic base at Rabaul on the northern coast of New Britain island (17 December 1943/8 August 1945).

Japanese forces landed near Rabaul in 'R' on 23 January 1942, capturing the town by February 1942, after which the harbour and town were transformed into a major Japanese naval and air installation. The Japanese heavily relied on Rabaul, which it used as the launching point for reinforcements to New Guinea and the Solomon islands group. Throughout the Solomon islands campaign, the 'Neutralisation of Rabaul' became the primary objective of the Allied effort.

After its capture by Admiral Shigeyoshi Inoue’s South Seas Force in February 1942, Rabaul was developed into a major fleet base by the Japanese, eventually becoming the most heavily defended Japanese position in the South Pacific. Rabaul’s strategic location, several airfields and large natural harbour made it the ideal staging base for ships, aircraft, troops and supplies during the New Guinea and Solomon islands campaigns. Men of the Japanese army dug many miles of tunnels as shelter from Allied air attacks. They also expanded the facilities by constructing barracks and support structures, and by 1943 there were about 110,000 Japanese troops in the area of Rabaul.

After the Japanese had lost their hold on Guadalcanal early in 1943, Allied forces began the push to the north-west along the chain of the Solomon islands group toward Rabaul. US Marine Raiders and US Army troops landed in the Russell islands group in 'Cleanslate' shortly after this, and a naval base was established there. The US forces then pushed the Japanese out of the New Georgia islands group in 'Toenails' from August 1943. The Japanese command had invested men and supplies in the construction of an airfield at Munda in New Georgia island, all of which proved to be a waste. Aircrsft of Lieutenant General George C. Kenney’s US 5^th Army Air Force made small attacks on Rabaul in October, and a major Allied air raid followed on 3 November. This latter raid destroyed 52 Japanese aircraft and five warships. Starting on 1 November, US Marines began their 'Cherryblossom' landing at Cape Torokina on Bougainville island, where several airfields were then constructed by the Allied forces. Most of Japan’s warships were withdrawn by 6 November.

With the major Japanese possessions around Rabaul captured, Allied air forces could then embark on the permanent neutralisation of Rabaul. And as part of efforts to isolate this base, US Army troops landed at Arawe on western New Britain on 15 December in 'Director', and the 1^st Marine Division landed at Cape Gloucester on 26 December 1943 in 'Backhander'.

As the major Japanese base in the South Pacific for the 4^th Fleet, Rabaul had been under continuous Allied air attack since the first raid by Consolidated Catalina twin-engine flying boats of the Royal Australian Air Force in January 1942. However a lack of resources and the very considerable distances involved (Rabaul was 500 miles/805 km from the nearest RAAF airfield at Port Moresby) ensured that these attacks remained small and sporadic for nearly two years.

As part of the 'Cartwheel' strategic plan, the US 5^th Army Air Force, the Royal Australian Air Force and the Royal New Zealand Air Force, all under Kenney’s command, began a sustained bombing campaign against the airfields and port of Rabaul in late 1943. The initial mission was delivered by 349 aircraft on 12 October 1943, but it could not be followed up immediately because of bad weather. A single raid by 50 North American B-25 Mitchell twin-engined medium bombers reached the target on 18 October. Sustained attacks resumed on 23 October, culminating in a large raid on 2 November.

After the first Japanese attempt to repel the Allied amphibious invasion of Bougainville island had been thwarted by US Navy surface forces at the 'Battle of Empress Augusta Bay', the Imperial Japanese navy sent a large naval force from Truk in the Caroline islands group to Rabaul for a second attempt. Lacking a comparable surface force of his own, Admiral William F. Halsey responded by ordering Rear Admiral Frederick C. Sherman to launch a dawn attack on the Japanese fleet at Rabaul using the air groups of the fleet aircraft carrier Saratoga and light aircraft carrier Princeton, followed an hour later by a 5^th Army Air Force raid by Consolidated B-24 Liberator four-engined heavy bombers. These attacks succeeded in damaging six of the seven Japanese cruisers present in Simpson Harbour, ending the Japanese threat to the Bougainville landings. A following raid on 11 November including the three carriers of Task Group 50.3 commanded by Rear Admiral Alfred E. Montgomery inflicted additional damage on the light cruiser Agano and shot down 35 Japanese aircraft.

The capture of Bougainville and Buka in 'Blissful' brought Rabaul within range of land-based US Navy and US Marine Corps tactical bombers, setting the stage for the pacification campaign to follow. Rather than attempt to capture the heavily fortified position, the Allies now decided to neutralise Rabaul by isolating it and eliminating its air power. The first air attack in the pacification campaign was planned for 17 December 1943. This was despatched from Torokina airfield on Bougainville and comprised 31 US Marine Vought F4U Corsair single-engined fighter-bombers, 23 RNZAF Curtiss P-40 single-engined fighter-bombers, 22 US Navy Grumman F6F Hellcat single-engined fighter-bombers, and a slightly smaller number of USAAF B-24 bombers. The attack did not receive a major response from the Japanese, so only seven Japanese fighters were lost. Three P-40s were lost, two of them with their pilots. A similar attack took place on 19 December, which cost the Japanese four aircraft, two credited to USMarine fighters.

The first major air attack took place on 23 December, and differed from previous attacks in the fact that the USAAF bombers went in first, with the fighters following. Some 40 Japanese fighters responded on this occasion, with 30 claimed destroyed by Allied fighters, though Japanese records do not match the Allied claims. Following another raid on 24 December, US Navy carriers attacked the Japanese force at Kavieng on New Ireland island in unison with an air raid on Rabaul. The US Navy carriers returned to Kavieng on 1 January 1944.

Throughout January 1944, the Japanese command devoted valuable carrier aircraft and invaluable carrier pilots to the defence of Rabaul. The seemingly hopeless situation into which the Japanese pilots were being fed was nicknamed 'the sinkhole in the Bismarcks' or the 'Bismarcks sinkhole'. January proved costly for the Japanese: 266 fighters were credited to US Marine Corps fighters and bomber gunners alone, not including the physical damage done to Rabaul’s land defences. In February, the Japanese command decided to withdraw all remaining Japanese aircraft and their crews from Rabaul. Between 70 and 120 Japanese aircraft flew from Rabaul to Truk, which had recently been raided by US Navy carrierborne aircraft, on the morning of 19 February. Their valuable mechanics attempted to leave Rabaul by ship on 21 February, but their ship, the Kokai Maru, was sunk by Allied bombers. This episode marked the end of Japanese air resistance to the Allies over Rabaul.

With Rabaul’s offensive capabilities neutralised, the Allies decided to forego a ground assault, electing instead to reinforce their foothold on the southern coast of New Britain against any potential Japanese counterattack while allowing the Rabaul garrison to 'wither on the vine'. Allied fighters and bombers continued to attack Rabaul through 1944 and 1945, and the regular attacks became known as 'milk runs' to Allied air crews. The only opposition over Rabaul was anti-aircraft fire, so attacking became a normality for Allied airmen and their maintenance crews. Eventually the Allied forces came to use Rabaul as a live-fire exercise to give aircrew some taste of combat before committing them elsewhere in the theatre.

The 'Neutralization of Rabaul' was ultimately a disaster for the Japanese. Most of their experienced carrier pilots had been lost over Rabaul, large numbers of their sorely needed aviation maintenance personnel were either lost during their attempted evacuation or trapped there, and the Japanese no longer had a base from which they could threaten the Allied presence in the Solomon islands group. By isolating Rabaul, a process completed by the seizure of the Admiralty islands and St Matthias islands groups in 'Brewer' and 'Beefsteak' in February and March 1944, the Allies effectively made its large garrison, which outnumbered the defenders of Okinawa, effectively prisoners of war without having to fight them, accommodate them or feed them. The last Allied air attack on Rabaul took place on 8 August 1945, only weeks before the Japanese formal surrender at the end of World War II.