In July 1941, only a few months before the attack, the United States, United Kingdom and the Netherlands had moved to freeze all Japanese commercial assets. The Allied nations were alarmed by continued Japanese aggression in China and the seizure of French Indochina from Vichy France in September 1940. Washington and London hoped that cutting off Japan's home islands from critical raw material imports would pressure Tokyo into backing down from its continued expansion. To the Japanese High Command, however, the idea of submitting to this pressure and curtailing what they believed to be Japan’s destined rise was unthinkable.
Japan, as an island nation, was heavily constrained by lack of resources. Going into the war, the nation imported 88 percent of its oil and was utterly dependent on raw material imports to sustain its industrial base. Unable to achieve self-sufficiency, and unwilling to capitulate, the Japanese had no alternative but to go to war and seize by force the resources they desperately required. Particularly vital to Japanese interests were the petroleum-rich Dutch East Indies — modern-day Indonesia — and the rubber plantations and tin mines of British Malaya. An Imperial push into Southeast Asia had the added advantage of cutting off the Burma Road, which ran north through modern Myanmar into China's Yunnan province. This key transit route had long sustained the Chinese in their struggle against Japan.
The Japanese war strategy, however, faced some massive constraints. The greatest limitation — one understood by the numerous Japanese naval officers who studied in the United States and United Kingdom — was the small size of Japan's industrial base compared to that of most Allied nations, particularly the United States. Admiral Isoroku Yamamoto, commander-in-chief of the Imperial Japanese Navy Combined Fleet, feared that given enough time the United States would marshal its resources and create an overwhelming force to crush Japan. With this limitation, Japanese leaders deduced they could not win a war of attrition against the much larger United States.
Japan's second disadvantage was its shortage of immediately available military resources. Although Japan's navy had avoided being totally bogged down in the Sino-Japanese war, the Imperial Japanese Army was already heavily committed to combat in China. Tokyo also needed to maintain the sizable Kwantung Army in Manchuria as a hedge against Soviet forces, and to invade Siberia if Germany managed to weaken Moscow. These constraints left the Japanese army with only 11 out of its 51 divisions available for the Pacific War. This meant that the Imperial war strategy relied even more heavily on maritime forces than would have been the case taking only geographic and topographic considerations into account.
The resulting Japanese war strategy hinged on massive initial blows that would surprise Allied fleets and air forces at port or in vulnerable airstrips. This would give Japan the maritime and air power advantage to rapidly seize its objectives and create an extended and heavily defended perimeter to protect both the home islands and Japan's newly acquired overseas resources before the Allies had a chance to recover. The Japanese could then present such a formidable and costly defensive line to the Allies that they would accept Japan’s gains and sue for peace.
The Fall of Asia
The Japanese conquest of Asia and the Pacific campaign that followed was initially an overwhelming success. Repeatedly underestimated by its enemies and often outnumbered, the disciplined, highly trained Japanese forces defeated American, British, Australian and Dutch forces as well as their local allies. The sheer expansion of Japanese territory was immense. Six months after Pearl Harbor, the Japanese Empire stretched from Manchuria in the north to New Guinea's jungle-clad Owen Stanley Range in the south. In the west the empire began at the borders of India's Assam and continued to the Gilbert Islands in the South Pacific. The Japanese Navy General Staff even debated whether they should invade Australia, though the army's heavy commitment in China nixed this plan — Tokyo barely had the forces to defend the territory it had already acquired.
By June 1, 1942, the Allies were in disarray and Japan held several key advantages. It now had the leeway to fortify its newly seized territories and to create a formidable, deep defense. And, unlike virtually all of its counterparts, the Imperial Japanese Navy was still largely intact. From this position of strength the Japanese could, in theory, stand firm along the outer perimeter and, when needed, dispatch maritime power to reinforce their forces or strike at the largely depleted U.S. fleets operating against them.
As long as the Imperial Japanese Navy remained dominant, it could continue to engage the Allied naval forces assembling against it, thereby further postponing any Allied strategic offensive. But Japan had two major problems. The United States was already mobilizing its massive industrial base for a military build-up — the launch of the first of the Essex Class carriers was scheduled for July 1942. Japan's shipbuilding industry could not keep pace and one significant naval defeat could mean that Japan would lose the crucial initiative. At the same time, Japanese forces were finding themselves stretched thin across a vast battlespace — especially the army.
The Empire Unravels
This overstretch ultimately led to the demise of Japan's imperial ambitions. In early June 1942, Japan lost four fleet carriers and accompanying elite aircrews at the battle of Midway. The Japanese still had a massive fleet that included a number of battleships and carriers, but they had lost the strategic initiative and could no longer dictate the pace of events as before.
With this shift, U.S. carrier fleets and amphibious forces could transition to the offensive. Because of their long perimeter, the Japanese had to disperse their forces across a broad frontage. They could not know where the Allies would strike next and could never marshal the required forces. Because of this, the Japanese almost always fought outnumbered. They could and did send maritime forces toward Allied strike points, but such moves required time, and ate into limited oil stockpiles. These stockpiles of essential fuels, oils and lubricants were already steadily declining because of a successful U.S. submarine campaign. Dispatching any forces also made Japan's limited maritime component vulnerable to expanding — and increasingly predatory — Allied fleets.
Possessing the combat power and economic might to defeat Japan across multiple locations, the United States began an island hopping campaign. This strategy neutralized Japanese defenses because the Allies could simply isolate and bypass many of the fortified Japanese positions and islands. Through the Guadalcanal campaign, and culminating in the "Great Marianas Turkey Shoot" during the Battle of the Philippine Sea, the Allied fleets gained naval dominance. The Imperial Japanese Navy would continue to make valiant efforts throughout the rest of the war in engagements such as the Battle of Leyte Gulf, but by June 1944, the tides had fully turned. The Allies gained the upper hand through a combination of island-hopping and the acquisition of fearsome new weapons such as the B-29 Superfortress, which could bomb the Japanese home islands over extended ranges.
In retrospect, the demise of Imperial Japan seems almost inevitable once the Allies demonstrated the resolve necessary to win a long battle with Japan, even in the face of the high casualties. The mismatch in the industrial capacity and military capability of the two sides was simply too great. In gross domestic product alone the United States was five times the size of Japan at the time of Pearl Harbor. Even a Japanese success at Midway would have only postponed the inevitable. The U.S. acquisition of the atomic bomb only quickened Japan's inevitable defeat. Unlike the Allies who could and did sustain multiple disastrous defeats and still hope to regain the strategic initiative, all it took was one critical naval defeat to reveal the weakness inherent in Japan's overreach.