In the spring of 1942, following their rapid successes during the early months after their entry into the war, the Japanese were ready to extend their control over the Coral Sea by capturing Port Moresby in New Guinea; thus isolating Australia from Allied help and opening the way for further advances in the south-west. The battle which ensued was the first naval battle to be fought entirely by aircraft; no ship on either side made visual contact with the enemy.
The Japanese had made great gains in the vast Pacific Ocean. The conquest of the Philippines, Burma, Malaya and the Dutch East Indies had cost the Japanese Navy only 23 warships and none had been larger than a destroyer. Sixty seven transport ships had also been lost. The Japanese naval command had expected far greater losses, therefore, encouraged by such success, they looked to expand still further in the Far East. However, the senior officers in the Japanese Navy disagreed on what was the best way forward. The very success which the Japanese had achieved in implementing their initial war plans had raised a fresh series of questions in the minds of those responsible for shaping Japanese naval strategy. And had caused a fierce and prolonged debate among the higher echelons of the Japanese naval command.
The Naval General Staff, headed by Admiral Nagano, advocated either an advance westward against India and Ceylon, or a thrust southward towards Australia. Admiral Yamamoto and the staff of the Combined Fleet, on the other hand, argued that a prolonged struggle would be fatal to Japanese interests, and regarded the first priority as being the destruction of the United States aircraft -carriers in the Pacific, to maintain Japanese security in the area. To this end, they urged early operations against Midway, to the eastward, seeing this as necessary for an attack on Hawaii. With the presence of the Combined Fleet in Hawaiian waters to support an invasion, the United States fleet would certainly be drawn out into a decisive battle, and could be dealt with before the Allies brought their emerging superior resources to bear against Japan.
The Japanese army, with its eyes on the Asian mainland and on Russia, objected to committing the large numbers of troops needed for the Naval General Staff's plans, and forced the latter to work out a more modest scheme. This involved moving from Rabaul and Truk, where Japanese forces were already firmly entrenched, into Eastern New Guinea, and down the Solomons and New Hebrides to New Caledonia, the Fijis, and Samoa.
In theory, the formulation of Japanese strategy was the responsibility of the Army and Navy General Staffs, operating jointly as sections of Imperial General Headquarters. In practice, however, the ability of the Combined Fleet to influence strategy had been demonstrated by Yamamota's insistence on the Pearl Harbour operation, despite the opposition of the General Staff. Subsequent events served to reinforce that influence.
It was the Americans who forced the hand of the Japanese. On the 18th April, the Doolittle raid on Tokyo, launched from the aircraft-carrier Hornet, inevitably strengthened Yamamoto's case for the Midway operation, particularly in the failure to keep the capital itself immune from bombing attacks.
The opposition of the General Staff promptly vanished. By 5th May, Admiral Nagano, Chief of the Naval General Staff, and acting in the name of the Emperor, issued Imperial General Headquarters Navy Order 18 which directed Yamamoto to 'carry out the occupation of Midway Island and key points in the Western Aleutians in cooperation with the army'. The operation to take place early in June.
However, the Japanese had decided on a course of action that spilt their forces. The attack on New Guinea had already started and could not be called off as it was too far advanced. Therefore, Yamamoto could not call on all the forces he might have needed for an attack on Midway Island as some Japanese forces were concentrated in the Coral Sea to the south-east of New Guinea. Thereby forcing upon the Japanese two concurrent strategies which were destined to over extend their forces. The plan for the impending Operation 'MO' was based on simple premises, but was over elaborate in the detail by which it was to be carried out. In fact this plan was too complicated, revealing a typical weakness which was evident throughout the war. It demanded a level of tactical competence which the Japanese did not possess. The division of forces in the plan might well be fatal to its prospects of success, should the Japanese meet a determined enemy when they were not in a position to concentrate and coordinate the separate units effectively.
The occupation of Port Moresby had originally been scheduled for March, but the appearance of American carrier forces in the south-west Pacific had caused the Japanese to postpone the operation until early in May, so that the V Carrier Division of the Nagumo force, then returning to Japan after the Indian Ocean operations, could be used to reinforce the IV Fleet at Truk and Rabaul. The V Carrier Division, under Rear-Admiral Hara,