Operation Cartwheel (30 June 1943- January 1944) was the name given to a series of interlocked invasions in New Guinea, New Britain and the Solomn islands originally designed as preparation for the conquest of the Japanese base at Rabaul, but that eventually led to the isolation of that base.
The Elkton III plan laid down a series of six interlocking operations for MacArthur's South West Pacific Command (Operations I, II and II) and Admiral Halsey's South Pacific Command (Operations A, B and C). These were eventually implemented as five major operations, beginning on 30 June 1943 and ending in the spring of 1944.
The conquest of Rabaul was originally part of the overall Allied plan. By July 1943 the Joint Chiefs of Staff were looking at bypassing that strong Japanese base instead of conquering it. The idea was approved by President Roosevelt in August and the new plan officially reached MacArthur's HQ on 17 September. This didn't have much impact on the Cartwheel operations, but it did help move the Admiralty Islands operation forwards.
Both commands would follow the spirit of Elkton III, but not all of the details. MacArthur made the fewest changes, mainly in the order of attacks, while the planned attack on Madang wasn't needed. The biggest change in his area was the invasion of the Admiralty Islands, which was brought forward so that it overlapped with the Elkton Plan.
Halsey made more changes to his timetable. The main invasion of New Georgia, timetabled for Operation B, ended up as part of the first phase of operations. The plans for Bougainville were totally altered. The original aim had been to take the Shortland Islands, off the southern tip of the island, and Buin, in southern Bougainville, as part of Operation B. Operation C would see the capture of Kieta on the east coast of the island and the neutralisation of the Japanese airfields at the southern end of Buka Island, off the northern tip of Bougainville. Instead the decision was made to land at Empress Augusta Bay, on the almost undefended west coast of Bougainville, and built new airfields in that area. The rest of Bougainville would be neutralised by air power.
Operation I - Operation Chronicle
Operation I was implemented as Operation Chronicle. This involved the invasions of Woodlark Island and Kiriwina in the Trobriand Islands. These islands hadn't been occupied by the Japanese, and the occupation went without too many problems. On the same day the Allies landed at Nassau Bay, south of Salamaua, in preparation for the second phase of the offensive.
Operation A - Operation Toenails
The original plan for Operation A was for a minor attack in either New Georgia or Santa Isabela. The aim was to pin down the Japanese aircraft in the area and prevent them from interfering elsewhere. The main invasion of New Georgia was originally timetabled for Operation B.
Halsey decided to bring the main attack on New Georgia forward to Operation A. The new attack was given the codename Operation Toenails, and the main aim was the capture of Munda, the major Japanese base at the western tip of the island. The invasion began on 30 June, with landings at Rendova, close to Munda, and secondary landings at the eastern end of the island. The main part of the invasion was the battle of Munda, which lasted from 2 July to 5 August. This was followed up mopping up operations, partly on the coast near Munda and partly on nearby islands. The fighting on the mainland finished at the end of August, and on the outlying islands by late September.
Operation II - Operation Postern
Operation II called for the occupation of the Japanese bases on the north-eastern coast of New Guinea, from Lae and Salamaua in the Huon Peninsula, to Finschhafen on the Huon Peninsula and then on to Madang, north of Astrolabe Bay. Although the overall aim was achieved, the order of events rather differed from the timetable in Operation II. D-Day for Postern was set as 4 September 1943, but the fighting around Salamaua actually began with the landings at Nassau Bay on 30 June 1943, and continued at a lower level across July and August.
The main Salamaua- Lae campaign began when Australian troops landed east of Lae on 4 September. This was followed by a parachute drop at Nadzab on 4 September, and the two forces then began to converge on Lae. The Japanese decided to evacuate the area, and after some fighting Salamaua was captured on 11 September and Lae on 16 September.
The campaign then split in two. The Huon Peninsula campaign saw Australian troops with US support fight their way around the coast of the peninsula. At the same time other Australian troops advanced west up the Markham Valley into the Ramu Valley, and were then involved in heavy fighting in the Finisterre Mountains. Finschhafen, on the tip of the peninsula, fell in October. The Australians then defeated a Japanese counterattack and began to make slow progress along the north coast. To the west their countrymen in the Finisterre Range ended up fighting a difficult battle on Shaggy Ridge, where the Japanese held up their attempts to cut north across the mountains to the north coast. The key moment in both campaigns came when US troops landed at Saidor, on the north coast, isolating the Japanese troops left on the Peninsula. General Adachi decided to order a retreat to Madang, and ordered his men to pull back from Shaggy Ridge. By April the Allies had met up at Astrolabe Bay, on the north coast. The Japanese them decided to evacuate Madang, which fell to the Allies on 24 April 1944.
Operation B - Operation Cherryblossom
The second stage of Cartwheel in the South Pacific, Operation B, originally had three objectives. The foothold on New Georgia that was to have been established in Operation A was to be expanded to complete the conquest of the island. The Japanese base at Faisi in the Shortland Islands, to the south-east of Bougainville, was to be captured. Finally the invasion of Bougainville itself was to start with an attack on the strong Japanese base at Buin on the south-east coast. This fighting was to begin five and a half months into Cartwheel - mid November 1943, and last to the end of December.
The first part of the plan had already been superseded by the decision to carry out a full-scale invasion of New Georgia as part of Operation A. The key Japanese base at Munda finally fell on 5 August, after an unexpectedly hard fight, and the clearing up operations were over by the end of September.
The second part of the plan was altered when Halsey's planners suggested bypassing the strongest Japanese positions and focusing instead on the undefended west coast of Bougainville. The Japanese bases elsewhere on the island could be dominated from new airfields built around this beachhead. This new plan was adopted, as Operation Cherryblossom. On 1 November 1943 American troops landed at Empress Augusta Bay, on the west coast, and quickly overcame very limited Japanese resistance. Although the Japanese believed this to be a diversion they still launched an immediate counterattack, using troops from Buin and Rabaul. This attack had been defeated by 11 November.
The Japanese then began to prepare to make a more powerful attack on the American beachhead. This finally came in March 1944 and put the beachhead under severe pressure for much of the month, but was eventually defeated. Operation B had been timetabled for mid November 1943, Cherryblossom began just ahead of schedule, while the Japanese counterattack was defeated just after the original time allocated to Operation C.
Operation III - Operation Dexterity
The final major part of Operation Cartwheel was the invasion of western New Britain, Operation Dexterity. This began with a diversionary landing at Arawe on the south coast on 15 December. The main landings came at Cape Gloucester, on the north-western tip of the island, on 26 December. Both areas were secure by early in 1944. The Americans then advanced east towards the Willaumez Peninsula, which jutted out from the north coast. After a final amphibious landing at Talasea on 6 March 1944 the fighting on New Britain died down.
The original Cartwheel plan had called for attacks on Kieta, on the eastern coast of Bougainville, and Buka Island, at the northern end of Bougainville, under the codename Operation C. They were to start seven months into Cartwheel, or January 1944. These attacks were cancelled.
While Cartwheel was still underway the Allies attacked the Admiralty Islands, landing on Los Negros on 29 February 1944. This campaign helped close the trap on Rabaul, and also forced the Japanese to evacuate Madang. The attention then moved west, towards Hollandia and Aitape. These key Japanese bases were attacked in April 1944 as Operation Reckless. This helped isolate the remaining Japanese troops in eastern New Guinea, and was also a key step in the advance towards the Philippines. In the meantime Rabaul was left to wither on the vine, and was the target of repeated Allied air attacks for the rest of the war.