As it happened, the Japanese commander, Major General Masutaro Nakai of the 20th Division, had ordered a sizeable force to move to Kaiapit under the command of Major Yonekura Tsuneo. Yonekura's force included the 9th and 10th Companies of the 78th Infantry Regiment, the 5th Company of the 80th Infantry Regiment, a heavy machine-gun section, a signals section and an engineer company—a total of about 500 troops. From Kaiapit it was to threaten the Allied position at Nadzab, creating a diversion to allow the Japanese garrison at Lae time to escape. The main body left Yokopi in the Finisterre Range on 6 September but was delayed by heavy rains that forced the troops to move, soaking wet, through muddy water for much of the way. Only the advance party of this force had reached Kaiapit by 18 September, by which time Lae had already fallen. Yonekura's main body, moving by night to avoid being sighted by Allied aircraft, was by this time no further from Kaiapit than King, but had two rivers to cross. Since both were heading for the same objective, a clash was inevitable.
