The Japanese Yokoyama Detachment, which was formed from approximately 900 infantry and engineers, landed at Buna on 21 July. The South Seas Force, led by Major General HORII Tomitarō, arrived later and pushed the Australian troops back to Isurava. The Japanese offensive intensified from 28 August and had reached Ioribaiwa by early September, where the lights of Port Moresby were visible.

However, the Japanese troops were extremely weakened by extended fighting and insufficient supplies. The Australian commander, Brigadier Eather, ordered a strategic withdrawal to Imita Ridge to await the Japanese offensive. Japanese army commanders, facing a counter-attack from American troops at Guadalcanal in the Solomon Islands, ordered HORII to withdraw his troops to Buna on 14 September.

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