Pearl Harbor and Nanshin-ron: The Context for the Beginning of the Southeast Asia Campaign

by Jack Bradley 

"Nanshin-ron," Southern Expansion doctrine, was proposed by the Japanese Navy to solve Japan's resource crisis–of which the most critical resource was oil. The U.S. had just imposed an oil embargo, and as the U.S. was the supplier of 80% of Japan's oil, the Japanese economy was now effectively being held hostage by America. As a result, Japan was forced to find another source of oil quickly. Therefore, the Japanese Navy proposed a blitzkrieg expansion throughout the Southeastern Pacific. One of the most important objectives was the large oil reserves in the Dutch East Indies (modern-day Indonesia).

A critical part of the attack would involve taking many European colonies of U.S. allies and invading the Philippines, a U.S. Territory, which would almost certainly result in the U.S. declaring war. The Japanese Imperial Navy advocated attacking the southeast because that is the campaign that would bring the Navy the most glory. At the same time, the army wanted to attack north into eastern Siberia. In 1939, the Soviet Union decisively defeated the Imperial Japanese Army in the battle of Khalkhin Gol. The battle shook the government's confidence that the army could defeat the Soviet Union in Siberia. 
​The only navies that could potentially challenge Japan in the Pacific were Great Britain and the United States at the time. The Royal Navy was already committed to the war in Europe, meaning there was little chance of being able to send a large enough force to challenge Japan in the Pacific. On the other hand, the U.S. Pacific Fleet had just been moved from the west coast to Pearl Harbor, which presented a significant challenge that Japan would have to defeat if it had any chance of success in the war to come. However, when the U.S. fleet was moved to Pearl Harbor, Admiral Yamamoto realized that the Hawaiian Islands were within range of Japanese carriers. If they could use those carriers to knock out the U.S. fleet at the very start of the war, Japan would have uncontested control of the Pacific, allowing their planned attack to go ahead smoothly. Without neutralizing the U.S.Pacific fleet, the Japanese would be forced to commit significant forces to the Western Pacific, the opposite direction they intended to attack, to defend against the inevitable American counter-attack.
While the preceding plan may seem logical at first glance, in actuality, it had a massive flaw that was overlooked by many high-ranking Japanese officers. The Americans massively outnumbered the Japanese in both population and industrial capacity. While the Japanese were aware of this, the plan they came up with if the Americans did not immediately sue for peace was to hope they gave up if they lost a certain amount of soldiers. The fact that the Americans might not just decide to quit after thousands had been killed in the surprise attack on Pearl Harbor does not seem to have been considered by the Japanese military. This left Japan in the position of fighting a very angry the United States with no strategic plan to win the war. Thus part of the blame for the Pacific War can be traced to the flawed logic of a plan that, in hindsight, had very little chance of succeeding. If the Japanese had realized what the Americans were actually capable of, the Japanese would not have started the war in the first place.





Hastings, Max. Inferno: The World At War, 1939-1945. 2011.

Toland, John. The Rising Sun: The Decline and Fall of the Japanese Empire, 1936-1945. 1970.