On 9 November 1942 Imperial General Headquarters (IGHQ) in Tokyo had established the Eighth Area Army Headquarters at Rabaul to coordinate Japanese Army operations in the Solomons and eastern New Guinea.

The 18th Army, organized that same day, was subordinated to the Eighth Area Army. Lt. Gen. Adachi Hatazo commanded 18th Army and was responsible for Japanese Army operations in eastern New Guinea. Adachi was sixtyone years old. His father had been an army officer, and two older brothers were major generals. Adachi graduated from the Japan Military Academy in 1910 and from the War College in 1922. He had commanded the 12th Infantry Regiment, 26th Infantry Brigade, and the 37th Infantry Division in combat. His operational experience was mainly in Manchuria and North China. In early 1943, IGHQ assigned the 20th, 41st, and 51st Infantry divisions to Adachi's command.

The 20th Infantry Division arrived in New Guinea from China in January 1943. Organized in 1917, this division had extensive combat experience in China dating from 1937. Its 78th and 80th Infantry regiments had distinguished combat records in North China, and the 79th Infantry Regiment had also gained extensive combat experience there. The 41st Infantry Division was organized in 1939. Its three infantry regiments, the 237th, 238th, and 239th, had widespread operational experience in North China and Inner Mongolia. The 51st Infantry Division, organized in 1940, had garrisoned the Canton region in South China from the summer of 1941 to November 1942, when IGHQ transferred the division to Rabaul.

All three divisions had fought against the Americans and Australians on New Guinea and had suffered serious losses. In January 1943, the 51st Infantry Division, for example, had spearheaded General Adachi's counterattack against Wait, where it had been repulsed by the Australian defenders. A convoy carrying reinforcements for the division's 115th Infantry Regiment met disaster when Allied aircraft attacked the ships. Most of the reinforcements perished when, after jumping from the sinking transports, they were strafed in the water by Allied fighters. The 51st and 20th Infantry divisions subsequently engaged combined American and Australian forces at Lae and Salamaua from June into September 1943 and at Finschhafen in late September and early October. The 238th Infantry Regiment, 41st Division, had participated. belatedly in the Salamaua fighting, and its sister regiment, the 239th, had counterattacked and driven back elements of the Australian 7th Division from Kesawai in December 1943.

The grueling jungle fighting had exacted a severe toll on the Japanese, in terms of both battle casualties and illness. Thus, in early 1944 Adachi was trying to rebuild the fighting strength of his units in anticipation of resisting future Allied attempts to retake eastern New Guinea. These preparations and Adachi's urgent signals for resupply and reinforcement were what Ultra revealed to the Allies.

As General Adachi reorganized his troops, Eighth Area Army Headquarters transmitted ambiguous orders that led to conflicting interpretations of Adachi's mission. On 27 February 1944 they sent two orders to Adachi. The first directed 18th Army to defeat the enemy troops advancing towards western New Guinea in a holding action at Madang. According to IJA doctrine, a holding action was designed to gain time in order to readjust to the changing operational situation, to deceive the enemy, or to delay the enemy's advance so that a stronger main defensive position might be established.

Simultaneously, Adachi received a second signal directing him, together with the Japanese 4th Air Army, to defeat the enemy advance in eastern New Guinea. Was Adachi's defense to be passive, simply holding a line, or active, counterattacking vigorously to disrupt the enemy's offensive preparations? Apparently higher headquarters left this decision to Adachi.

Moreover, the Japanese expected the next Allied attack to fall on Madang, particularly after MacArthur's seizure of the Admiralty Islands put Madang within range of Allied fighters. The Japanese used the rule of thumb that the effective fighter range for the continuous type of support required in Allied amphibious invasions was about 480 kilometers. Madang fell within that range, but Hollandia and Aitape were beyond it. General MacArthur had heretofore not used carrier-based fighters for tactical air support, so Japanese planners downgraded the immediate threat against Hollandia and Aitape because of their distance from Allied airfields. They concentrated 4th Air Army near Hollandia, presuming that it was beyond effective Allied fighter range and that the Allies would not attack the base with unescorted bombers. They overlooked the possibility that MacArthur might employ carrier-based aircraft not only against the major concentration of Japanese aircraft at Hollandia, but also in a close air support role at Hollandia and Aitape.

Consequently, defenses at both bases languished as the Japanese strengthened those areas where they believed MacArthur's next blow would fall. Based on these assumptions, Japanese staff officers concluded that MacArthur's next landing would occur between Madang and Hansa Bay. [16]

Accordingly, IGHQ ordered 18th Army to abandon Madang and to withdraw rapidly west of Wewak, but only after striking a "heavy blow" against the invaders. [17]

General Adachi deployed his 51st Division at Madang, his 20th at Bogia, and his 41st at Wewak. From this disposition he hoped to trap Allied invaders between Hansa Bay and Wewak in a pincer movement, a stratagem Ultra revealed to Allied planners. [18]

MacArthur's capture of the Admiralties also isolated Rabaul. As Eighth Area Army at Rabaul could no longer effectively control 18th Army, IGHQ assigned 18th Army and 4th Air Army to General Anami Korechika's Second Area Army, headquartered at Davao, Netherlands New Guinea. Anami's command was responsible for the defense of western New Guinea, a strategic area controlling the southern approach to the Philippines and the Caroline Islands.

The Japanese primary defense line, decided at an imperial conference on 30 September 1943, ran south of Timor and Tanimbar islands, through the western side of Geelvink Bay, thence to Truk, the major Japanese naval base in the Caroline Islands. Second Area Army's front line included Tanimbar and Aroe islands, extended to Sarmi, and then swung back through Sorong to Halmahera (see map 6).

General Anami ordered Lieutenant General Adachi to move 18th Army westward to Wewak, thence to deploy his forces to defend Wewak, Aitape, and Hollandia. The 18th Army's main strength, however, still remained east of Hansa Bay. In order to implement Second Area Army's orders, Adachi had to move his battle-depleted, poorly supplied, and scattered units 500 kilometers west, from Wewak to Aitape, and nearly 850 kilometers from Wewak to Hollandia. In mid-April 1944, during the redeployment, the elements of the 51st Division had just reached Wewak, the 20th Division was passing through the vast swamp between the Ramu and Sepik rivers, and the 41st Division was stretched out between Madang and Hansa. More than 30,000 Japanese troops were scattered east of Hansa, and 18th Army staff officers estimated that they would need another fifty days to get through the rugged terrain. [19]

The main reason for the slow pace was a lack of gasoline-powered transport. The 18th Army lacked both trucks for overland movement and barges for sea transport. A forced march was their last resort. Beyond the sheer physical demands of such a move, they made their trek under increasingly dangerous circumstances. Intensified Allied aerial activity made all movement hazardous.

While sudden tropical downpours would temporarily curtail the air threat, the rain would wash out jungle tracks and turn swamps into small lakes, further impeding the westward progress of the Japanese. Thus, at the time of the Allied landings at Hollandia and Aitape, 18th Army had not completed its major redeployment. Instead, its combat units were scattered along a 470-kilometer stretch of New Guinea coastline.