'Interlude' was the US undertaking to plan and make rehearsal landings at Aitape and to the west of Wakde in North-East New Guinea and Dutch New Guinea for the 'Tradewind' assault on Morotai island in the Halmahera islands of the Japanese-occupied Netherlands East Indies (4/6 September 1944).
Once the logistic, tactical and airfield development plans for 'Tradewind' had been completed, the various formation and unit commanders involved began to consider how to solve the final problems of staging the assault and also how best to undertake rehearsals to validate their planning and also to give the troops involved some experience of the type of operation to which they were committed. Unforeseen difficulties made it necessary to draw up intricate staging schedules, which resulted in the most complicated staging plan employed in the South-West Pacific Area since the interlinked 'Reckless' and 'Persecution' assaults on Hollandia and Aitape on 22 April 1944.
Initially, Lieutenant General Walter C. Krueger, commander of the 6th Army ('Alamo' Force), had hoped to stage the entire task force for 'Tradewind' at Maffin Bay, near Wakde and thus about 125 miles (200 km) to the west of Hollandia, but as early as 22 July the planners started to appreciate that the crowded conditions and inadequate facilities at Maffin Bay would make it impossible to execute their plan. The 6th Army’s logistics planners therefore recommended that staging be divided, with the task force headquarters, task force artillery, the 124th Regimental Combat Team of Major General John C. Persons’s (from September 1944 Major General Clarence A. Martin’s) 31st Division, and the task force reserve staging at Aitape, where all of these were already located. The rest of the 31st Division would stage at Maffin Bay, where no more than 1,000 men of the Allied air forces would also load. Most of the air force units would be staged at Hollandia or Biak, while various engineer units would be staged at Hollandia, Aitape, Maffin Bay, Sansapor and Finschhafen. Realising that no other solution was logistically possible, Krueger and Rear Admiral Daniel E. Barbey, commander of the 7th Amphibious Force tasked with the delivery and landing of the 'Tradewind' Task Force, approved the split staging plan.
In order to prepare the necessary airfields and associated facilities at Morotai on schedule, the staging and shipment of engineer units was planned so that most of these units would reach Morotai by the third day of the 'Tradewind' landing. Nearly all the rest of them were to arrive by the sixth day, and rear echelons of all were to close at Morotai by the sixteenth day.
By 3 September these staging plans, as well as the tactical and logistic plans for all air, naval, and ground units, had been completed. The bulk of the assault shipping and landing craft had reached the two principal staging areas, Aitape and Maffin Bay, on 2 and 3 September to begin loading and to undertake rehearsals.
The latter were carried out at Aitape on 4 September for the 124th RCT and smaller units scheduled for the assault on the White Beach on Morotai. The Shore Battalion, 534th Engineer Boat and Shore Regiment, for whose benefit the rehearsal was largely undertaken, came ashore in good order. Despite the fact that the battalion had had no previous assault experience, Major General Charles P. Hall, commanding the XI Corps and the 'Tradewind' Task Force, ended the rehearsal at mid-morning. By that time no bulk stores had been unloaded and many vehicles had not been sent ashore, but Hall terminated the rehearsal because it was difficult to find dispersal room for all the supplies and equipment which had already been unloaded, because there was a danger that some scarce equipment might be damaged and, finally, because much time would be needed to reload the ships. Even so, Barbey believed that none of these factors outweighed the value of complete rehearsal, particularly in light of the very considerable tonnage of supplies and equipment which was to be unloaded at Morotai during the assault phase. However, actual landing conditions at Morotai later indicated that more complete rehearsals might have had little value.
The Red Beach assault units carried out a rehearsal on the mainland of New Guinea to the east of Wakde island on 6 September. The landings again proceeded smoothly, and the rehearsal was therefore cut short. Some LST tank landing ships did not beach and others could not discharge all their vehicles because the rehearsal beaches were not particularly good. Many other vehicles were not unloaded because the 31st Division, like Major General Paul A. Mueller’s 81st Division earmarked for the 'Stalemate II' operation in the Palau islands group, lacked sufficient waterproofing material for both extended rehearsals and the assault.
The final loading of the 'Tradewind' Task Force took place generally without difficulty except at Aitape, where adverse surf conditions made it necessary to use LCT tank landing craft to ferry troops from the beach to ships lying offshore. At Maffin Bay most of the embarkation was carried out directly from the beach except that some troops were ferried from the shore to LCI infantry landing craft by DUKW amphibious trucks. Loading was completed at Aitape on 8 September, and that section of the assault force departed the area on the following day to rendezvous on 11 September with the Maffin Bay group. Departing Maffin Bay on 12 September, the convoy was joined on the next day by its covering force and supporting escort carriers.
To achieve as much secrecy as possible, the convoy was routed 40 miles (65 km) to the north of Biak island and thence top the north-west out of sight of the Asia and Mapia islands, where the Japanese were believed to maintain garrisons and radio stations.
Movement beyond Biak was practically without incident. There was one suspected contact with a submarine, but this could not be found, and an unexpectedly strong westerly current was encountered, forcing the convoy to reduce speed in order not to reach Morotai too early. Landfall was made as scheduled on the morning of 15 September. Neither the destination nor the approach of the attack force had been detected by the Japanese.