14-16 September 1942
At Ioribaiwa, forty kilometres from Port Moresby, the Australians assembled for battle their largest force so far in the fighting in Papua. The Japanese attacked and, for the last time, drove the Australians from their position...
Animated map of the action at Ioribaiwa Ridge
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Plans and forces
After his failure at Efogi Potts was replaced by Brigadier Porter. Porter was reinforced by an Australian Imperial Force brigade under Brigadier Eather who then took command of Maroubra Force for the action at Ioribaiwa. With the fresh 25th Brigade, Eather planned to attack the Japanese. He placed 21st Brigade which had fought at Efogi, with 3rd Battalion, in a defensive position on Ioribaiwa Ridge. He left one of his own battalions, 2/25th, in reserve behind them. The other two battalions of 25th Brigade were to outflank the Japanese, one on either side.
The Japanese struck first, as Eather was still deploying for his attack. Eather responded by calling off his offensive and adopting a defensive stance. The main body of the Nankai Shitai was still in the rear awaiting permission to recommence the advance on Port Moresby. The action at Ioribaiwa was fought by Kusunose's pursuit group, the same force that had fought at Efogi, plus the mountain artillery. In all the Japanese had about 1650 fighting men present.
The action at Ioribaiwa
On 14 September on the Australian left 2/31st Battalion, making in its own flanking move, encountered the Japanese 2/144 Battalion trying to do the same thing in the other direction. Neither battalion was able to gain any advantage over the other. In the Australian centre 21st Brigade, much reduced by three weeks fighting, was attacked by a half battalion of 3/144 Battalion. Kusunose had massed all his artillery, eight guns, on a ridge north of Ioribaiwa and from here they pounded the Australian centre. As had happened to 2/27th Battalion at Efogi, half of all the casualties in 21st Brigade were to Japanese artillery bombardment. The Australians held on and Kusunose's infantry in the centre made no progress. His attack on the Australian left was similarly unsatisfactory so, with the half battalion he had in reserve, he decided to try the Australian right flank.
This new Japanese move began on 15 September. It failed to find the Australian right flank because the Australian line was longer than Kusunose estimated, indeed longer than Eather had planned. When the first Japanese attack began the Australian battalions about to advance to find the Japanese flanks had, acting on Eather's orders, halted where they were at that time. By good fortune this placed the Australian infantry battalions in a rough line, over two kilometres long, along Ioribaiwa Ridge. The manoeuvring battalions in effect extended the line of 21st Brigade to the right and to the left. The Japanese half battalion looking for the Australian right flank struck the line well short of the flank, between 3rd Battalion and 2/33 Battalion. The Japanese managed to insert themselves there but, almost surrounded by Australians, it was all they could do to hold on. A counterattack by a half battalion of the Australian reserve, 2/25th, was unsuccessful and one by 2/33rd failed to find the enemy in the thick jungle.
By the morning of 16 September there was a stalemate. Eather's left was held up by 2/144, his centre was under constant artillery bombardment and on his right he was unable to eject the Japanese who had penetrated into his position. From Kusunose's viewpoint things were equally grim. His original flanking move on the Australian left had bumped into enemy where there should have been none. Unaware of the heavy casualties his artillery was inflicting on the Australian centre he saw only that he had gained no ground there. On the Australian right his second attempt at outflanking had also failed. Eye to eye, Eather was the first of the two commanders to blink. He requested and received permission to withdraw to Imita Ridge. By the evening of 16 September the Australians were safely in the new position.
Australian casualties were 49 killed and 121 wounded or five percent of the force on Ioribaiwa Ridge. The Japanese lost 40 dead and 120 wounded. With hindsight it is apparent that, unlike the previous encounters in the mountains, the Australians were not defeated but rather had fought the Japanese to a draw. However by withdrawing Eather showed that, ignorant of his significant numerical advantage, he saw himself as defeated. He offered a number of plausible reasons for pulling back to Imita Ridge but the central one can be found in his signal to his superior, Major General Allen: 'Do not consider can hold him (the Japanese) here'.