The American invasion of Japanese held Guadalcanal in August 1942 diverted Japanese attention from New Guinea and was of great assistance to the Australians in their defence of Port Moresby...
On 7 August 1942 the United States Navy landed a Marine Division on the islands of Guadalcanal and nearby Tulagi. Guadalcanal was one of the locations the Japanese had earmarked as part of their outer ring of defences and they had begun to build an airfield there. The Americans easily overcame the small number of Japanese present and planned to make the airfield their own. The Japanese in turn made a number of attempts to recover the airfield. At the same time the Americans reinforced their Marines. The resulting series of battles, which occurred at the same time as the Kokoda campaign, exercised a strong influence on it.
Initially the Japanese believed they had a satisfactory force to take Port Moresby and that Guadalcanal might easily be recovered. Then an Australian counterattack which briefly retook Kokoda on 9 August, suggested to them that the Australians might be stronger in Papua than they had anticipated. Within days General Hyakutake, 17th Army commander in Rabaul, was informed that the Americans at Guadalcanal were also much stronger than had been believed. Now faced with two large problems which had just days before seemed to him two small problems, Hyakutake decided, his resources being insufficient for both, to put the advance on Port Moresby on hold and retake Guadalcanal. That done he planned to reinforce Papua and take Port Moresby.
The Kawaguchi detachment, originally to go to Papua, was directed to land at Guadalcanal. In the naval Battle of the Eastern Solomons, on 23 August, the landing was foiled. As a result the Japanese again drew on Papuan resources to aid Guadalcanal. Almost all Imperial Japanese Navy aircraft were withdrawn from New Guinea to support a further, successful attempt, to land the Kawaguchi detachment. Japanese airpower remained focused on Guadalcanal to the end of the year.
Hyakutake was content to meanwhile push towards Port Moresby and take up a useful position on the south side of the crest of the Owen Stanley Range that would serve as a jumping off point when the attack on Port Moresby was resumed. However in mid-September Guadalcanal again influenced events in Papua. Kawaguchi's attack on the Americans there was a complete disaster. Horii in Papua, by then on Ioribaiwa Ridge forty kilometres from Port Moresby, was consequently ordered to withdraw.
In October the Japanese again reinforced Guadalcanal but their next, and largest, attempt to destroy the American force was repulsed. The effect upon operations in Papua was that Horii was ordered to fall back yet again, from the Eora-Templeton's position to beyond Kokoda. Caught in the act of withdrawing, by an Australian attack on 28 October, his defeat at Eora was more serious than it might otherwise have been.
By November Imperial Headquarters in Tokyo had still not given up the idea of recovering Guadalcanal but realised that they must now abandon any idea of resuming an offensive towards Port Moresby. The great Australian victory at Oivi-Gorari and the Australian-American advance along the coast towards Buna confirmed what Imperial Headquarters had already decided: If there remained any chance to retake Guadalcanal then all that could be done in Papua was to hold their base on the coast between Gona and Buna. It was not until the Japanese decided in December to evacuate Guadalcanal that significant land and air reinforcements were again sent to Papua. It proved to be too little, too late.
Whether Hyakutake made the correct choice in deciding Guadalcanal was more important than Papua is open to debate. What can be said is that had he decided otherwise the Australians along the Kokoda track and at Milne Bay would have faced far larger Japanese forces with strong air support.