'Cartwheel' was the US operational version of the final 'Elkton III' plan whereby the forces of Admiral William F. Halsey’s South Pacific Area and General Douglas MacArthur’s South-West Pacific Area commands were to co-operate in a double advance to isolate General Hitoshi Imamura’s 8^th Area Army, of General (Field Marshal from June 1943) Count Hisaichi Terauchi’s Southern Expeditionary Army Group, on the islands of New Britain and New Ireland in the Bismarck islands group (26 April 1943/20 March 1944).

The primary elements of 'Cartwheel' were the US 'Chronicle' landings on Woodlark island and Kiriwina, New Guinea; the US 'Toenails' landings on New Georgia island; the Australian and US 'Postern' assault on Lae in Papua New Guinea; the New Zealand 'Goodtime' landing on the Treasury islands; the US 'Blissful' landing on Choiseul island; the US 'Cherry Blossom' landing on Bougainville island; the US 'Dexterity' landings at Arawe, Cape Gloucester and Saidor and the capture of Tuvulu aerodrome, and the Australian, New Zealand and US assault on New Britain island.

The islands of the Bismarck archipelago are located to the north-east of New Guinea and the north-west of the Solomon islands group, with which they share a very difficult jungle-clad terrain and rain-swept tropical climate. German settlers began to reached the area in the 1860s and Germany annexed the islands in 1885. Australian troops expelled the Germans in September 1914 in the early stages of World War I, and Australia received a mandate over the islands from the League of Nations on 9 May 1921.

The islands' land area is about 19,200 sq miles (49730 km²), and the largest of the islands is New Britain with an area of 13,569 sq miles (35145 km²). There are superb anchorages at Seeadler Harbour in the Admiralty islands group to the north-west of New Ireland and at Rabaul on the north-eastern tip of New Britain, but only the latter had been developed to any extent by the end of 1941, with modest docking facilities and two airfields. There is also an anchorage at Kavieng on the north-western tip of New Ireland, which has an area of 2,859 sq miles (7404 km²), but it was overshadowed by Rabaul throughout the war. Before the advent of the Pacific War, the islands' chief economic strength was the export of copra, timber and turtle shells.

As well as being covered with jungle, the islands are very rugged, and include several active or dormant volcanoes. The anchorage at Rabaul is a flooded caldera, and in World War II resurgent domes in the harbour area periodically gave off quantities of steam and ash, and sulphurous fumes adversely affected the habitability of the many underground fortifications which the Japanese constructed during the war.

The islands receive their heaviest rains during the north-western monsoon of October to April, but there is no real dry season. Tropical diseases such as malaria were common, particularly on New Britain.

The local population consisted of Melanesian tribesmen, with some Polynesian blood, who had largely converted to Christianity by 1942. Many members of the local population were contracted as labourers, for the immigration of Asian labourers was prohibited.

The Bismarck islands group was scheduled for occupation in the earliest iterations of the Imperial Japanese navy’s plans for the 'Centrifugal Offensive' to become part of the outer defence line against the expected US offensive drive to the west as outlined on the 'Orange' interwar colour-coded war plan.

Rabaul was very clearly the strategic key to the Bismarck islands group. The Japanese recognised the value of the harbour, and seized it in 'R' with forces from Truk atoll early in the Pacific War. Air attacks began on 4 January 1942 and elements of Major General Tomitaro Horii’s South Seas Detachment began to land on 23 January, rapidly driving back the 1,390 men of the defending Australian 22^nd Battalion ('Lark' Force) and taking the town and airfields. With Rabaul secured, the Japanese occupied the remainder of the Bismarck islands group without difficulty. Kavieng was taken in 'O' (i) on the same day as Rabaul, Bougainville was occupied on 30/31 March in 'Bo', and the Admiralty islands group was occupied on 8 April.

The Allied strategy in the South-West Pacific Area was initially focused on the recapture of Rabaul. General Douglas MacArthur, the commander-in-chief of the South West Pacific Area, envisaged the 'Cartwheel' two-prong counter-offensive in which one prong would drive to the north-west from the Solomon islands group and the other to the north across the Dampier and Vitiaz Straits from New Guinea to New Britain. These operations began with the operations to secure Guadalcanal in the Solomon islands group in 'Watchtower' starting on 7 August 1942, and to clear the north-eastern coast of New Guinea around Buna starting on 19 November 1942. Both undertakings proved to be far more difficult than had been foreseen, becoming major battles of attrition which lasted for months: the Buna area was not secured until 22 January 1943 and Guadalcanal until 9 February 1943.

At at the Pacific Military Conference of March 1943 in Washington, DC, MacArthur’s representative, Major General Richard K. Sutherland, the South-West Pacific Area’s chief-of-staff, presented the 'Elkton III' revised plan for the recapture of Rabaul, a plan which envisaged the seizure of the Huon peninsula in New Guinea and Munda on New Georgia, followed by the seizure of points in western New Britain and Bougainville. The Allies could then take Kavieng, if necessary, before the final assault on Rabaul. The strength of the Japanese forces in the area was estimated at some 85,000 men and 383 aircraft, with another 11,000 men, 250 aircraft, and the main strength of the Combined Fleet available for immediate reinforcement. In the longer term, the Japanese could dispatch another 615 aircraft and something in the order of 10 to 15 divisions into the area if shipping could be found. Japanese records show that this estimate was fairly accurate, and that the shipping available was about 300,000 tons to which perhaps another 100,000 tons could have beeen added. MacArthur demanded another five divisions and a tripling of the air strength in the theatre in order to carry out his plan.

The Washington planners rejected any reinforcements beyond two or three divisions and a small number of aircraft, and the plan had therefore to be trimmed. The final directive, which was issued on 28 March 1943, was based on Allied advances as far as the Huon peninsula, the western end of New Britain and Bougainville by the end of 1943. Overall command was to be vested in MacArthur, with whom Admiral William F. Halsey, commander-in-chief of the South Pacific Area, would co-operate. Fortunately, there was enough mutual respect between the two men to make the plan work.

Controlled directly by MacArthur’s South-West Pacific Area command, the US forces for this undertaking were grouped as 'Alamo' Force under the command of of Lieutenant General Walter C. Krueger, whose US 6^th Army constituted the major strength of 'Alamo' Force. The southern portion of 'Cartwheel' involved an advance along the coast of New Guinea as far to the west as Madang against the forces of Lieutenant General Hatazo Adachi’s 18^th Army, and the northern portion an advance to the north-west along the chain of the Solomon islands group against the forces of Lieutenant General Harukichi Hyakutake’s 17^th Army (operational command passing from the South Pacific Area to the South-West Pacific Area between Choiseul and Bougainville islands) before a wide outflanking descent was made on the Admiralty islands and St Matthias islands, to the north-west of Kavieng in New Britain, during March 1944 to seal the 8^th Area Army's last possible route of escape or reinforcement.

The Japanese forces thus isolated were to be left essentially undisturbed 'to wither on the vine' for the rest of the war.

Strategic direction of 'Cartwheel' was entrusted to MacArthur as the Supreme Allied Commander in the South-West Pacific Area (SWPA), and involved primarily US and Australian forces as well as smaller Dutch and New Zealand elements. US and New Zealand forces were involved secondarily under the other Allied command in the Pacific, namely the Pacific Ocean Areas, in their ongoing thrust to the north-west through the Solomon islands group toward Bougainville.

Major General Tomitaro Horii’s South Seas Detachment(essentially a reinforced brigade group of Lieutenant General Hiroshi Takeuchi’s 55^th Division) and elements of Admiral Shigeyoshi Inoue’s 4^th Fleet had taken Rabaul on New Britain, in the Territory of New Guinea, from Australian forces in February 1942 and subsequently turned it into their major forward base in the South Pacific, and the main obstacle to advance in the two Allied theatres.

Early in 1943 MacArthur requested five more US Army divisions with which to launch a direct attack on Rabaul, although he could have used Australian forces already at his disposal, but on 28 March 1943 the Joint Chiefs-of-Staff issued a directive for MacArthur instead to isolate Rabaul by means of three thrusts on mainland New Guinea and the island archipelago round the Bismarck Sea to the north-east of New Guinea. For this he was to receive reinforcement in the shape of Krueger’s newly formed 6^th Army (known initially as 'Alamo' Force).

The Japanese and Allied forces available in the South-West Pacific Area in the third quarter of 1943 as 'Cartwheel' began in earnest were as follows.

For the Japanese army, Lieutenant General Hitoshi Imamura’s 8^th Area Army, with its headquarters at Rabaul, controlled Lieutenant General Haruyoshi’s 17^th Army with its headquarters in the Shortland islands and controlling Lieutenant General Masatane Kanda’s 6^th Division on Bougainville; Lieutenant General Hatazo Adachi’s 18^th Army with its headquarters at Madang and controlling Lieutenant General Heisuke Abe’s 41^st Division at Wewak, Lieutenant General Shigeru Katagiri’s 20^th Division at Madang and Lieutenant General Hidemitsu Nakano’s 51^st Division at Lae; the 6^th Air Division with about 240 aircraft; the 38^th Division on New Britain; and Major General Iwao Matsuda’s 65^th Brigade on New Britain.

For the Japanese navy, Vice Admiral Junichi Kusaka’s South-East Area Fleet, with its headquarters at Rabaul, controlled Vice Admiral Gunichi Miakwa’s 8^th Fleet with its headquarters in the Shortland islands and controlling Major General Minoru Sasaki’s South-Eastern Detachment on New Georgia island with the 13^th Regiment and 229^th Regiment, Rear Admiral Minoru Ota’s 8^th Combined Special Naval Landing Force on New Georgia island and Rear Admiral Minoru Katsuno’s 7^th Combined Special Naval Landing Force on Santa Isabel island; Kusaka’s 11^th Air Fleet at Rabaul with about 300 aircraft; and warships including one light cruiser, eight destroyers and eight submarines.

Japanese reinforcements in the 'Cartwheel' period were the 7^th Air Division, which arrived in July 1943, and Lieutenant General Kumaichi Teramoto’s 4^th Air Army, which arrived in August 1943.

For the Allies, MacArthur’s South-West Pacific Area command, headquartered at Port Moresby, comprised Lieutenant General Walter C. Krueger’s 'Alamo' Force (41^st Division, 2^nd Engineer Special Brigade, 158^th Infantry, 112^th Cavalry, 503^rd Parachute and 12^th Marine Defense Battalion), Lieutenant General Sir Thomas Blamey’s Allied Land Forces (Australian 3^rd and 9^th Divisions), Lieutenant General George C. Kenney’s Allied Air Force with Kenney’s own US 5^th AAF (Brigadier General Roger M. Ramey’s V Bomber Command and Brigadier General Paul B. Wurtsmith’s V Fighter Command), Air Vice Marshal William D. Bostock’s Royal Australian Air Force (Hewitt’s No. 9 Group); and Vice Admiral Arthur S. Carpender’s Allied Naval Forces with Rear Admiral Daniel E. Barbey’s Task Force 67 (Amphibious Force).

The forces of Halsey’s South Pacific Area, headquartered at Nouméa on New Caledonia island and reinforced with ships of Admiral Chester W. Nimitz’s Pacific Fleet as and when necessary, comprised Halsey’s own 3^rd Fleet controlling Rear Admiral Richmond K. Turner’s TF31 Amphibious Force, which was loaned to MacArthur when needed, with eight destroyers, 36 PT-boats, nine troop-carrying destroyer conversions, three cargo ships, three transport ships, 15 tank landing ships and 39 infantry landing craft; Vice Admiral Aubrey W. Fitch’s TF33 (Air Force South Pacific) with about 258 fighters, 193 medium and light bombers, 82 heavy bombers and 12 flying boats, and including Rear Admiral Marc A. Mitscher’s Solomon Islands Air Force at Henderson Field on Guadalcanal with the 1^st and 2^nd Marine Air Wings; Lieutenant General Millard F. Harmon’s Army Forces South Pacific with about 275,000 men (Americal Division, 25^th Division, 37^th Division with no combat experience and 43^rd Division with limited combat experience), Major General Nathan F. Twining’s US 13^th AAF, and Major General Clayton B. Vogel’s I Marine Amphibious Corps with the 1^st Marine Division on loan to the SWPA, 2^nd Marine Division, 3^rd Marine Division, 1^st Marine Raider Regiment, six marine defence battalions and the 1^st Marine Parachute Regiment.

The 'Cartwheel' plan indicated the need for 13 sub-operations and suggested the appropriate scheduling of these. In the event three of the sub-operations (those directed at Rabaul, Kavieng and Kolombangara) were removed as too costly and indeed unnecessary in operational as well as strategic terms, so only 10 were implemented. These latter, with their dates of landing and their invasion units, were 'Chronicle' on Woodlark island by the 112^th Cavalry and Kiriwina (Trobriand islands group) by the 158^th Regimental Combat Team on 30 June 1943; 'Toenails' on New Georgia by the 43^rd Division on 30 June 1943 (and including Segi Point on New Georgia by the 4^th Marine Raider Battalion on 21 June 1943, Rendova island off New Georgia by the 43^rd Division’s 169^th and 172^nd Regimental Combat Teams on 30 June 1943, Zanana on New Georgia by the 169^th and 172^nd Regimental Combat Teams on 5 July 1943, Bairoko on New Georgia by the 4^th Marine Raider Battalion on 5 July 1943, Arundel island by the 172^nd Regimental Combat Team on 27 August 1943, and Vella Lavella by the 35^th Regimental Combat Team of the 25^th Division and the New Zealand 3^rd Division on 15 August 1943); 'Postern' on Lae in New Guinea by the Australian 7^th and 9^th Divisions and the 503^rd Parachute Infantry on 5 September 1943; 'Goodtime' on the Treasury islands by the New Zealand 8^th Brigade on 27 October 1943; 'Blissful' on Choiseul island by the 2^nd Marine Parachute Battalion on 29 October 1943; 'Cherryblossom' on Bougainville by the 3^rd Marine Division and 37^th Division on 1 November 1943; 'Michaelmas' on Saidor in New Guinea and in New Britain ('Director' on Arawe on New Britain by the 112^th Cavalry on 15 December 1943, 'Backhander' on Cape Gloucester by the 1^st Marine Division on 26 December 1943, and Saidor by the 32^nd Division on 2 January 1944); 'Brewer' on the Admiralty islands group by the 1^st Cavalry Division on 29 February 1944); and 'Beefsteak' on Emirau island in the St Matthias islands group by the 4^th Marines on 20 March 1944.