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IB DP History Study Notes

3.1.6 Towards World War: Tripartite Pact to Pearl Harbor

Delving deeper into the complex series of events leading Japan into World War II, we'll explore the intricate balance of international relations, strategic motivations, and the pressing economic realities that informed Japan's decisions.

The Formation and Implications of the Three Power/Tripartite Pact

Origin of the Pact

  • The geopolitical scenario of the late 1930s was dominated by the aggressive expansionist policies of Germany in Europe and Japan in Asia. To counterbalance the major Allied powers and further their own interests, these nations sought alliances.

Details of the Tripartite Pact

  • The Tripartite Pact was signed in Berlin on 27 September 1940 by representatives from Germany (Joachim von Ribbentrop), Italy (Galeazzo Ciano), and Japan (Saburō Kurusu).
  • The pact explicitly stated that if any of the signatories were attacked by a power not currently involved in the war, the other two would come to its defence.

Implications of the Pact

  • Strategic Implications: The key strategy behind this pact was to discourage the US from supporting Britain or joining the war. Japan also saw it as a counterbalance against the increasing threat from the Soviet Union.
  • Economic Implications: For Japan, the pact meant stronger economic ties with Germany and Italy, offering an alternative to its trade relations with the US, which were deteriorating.
  • Political Implications: The alliance gave Japan a global stature. It portrayed Japan as part of a formidable bloc against the Allied powers.

Events Leading to the Attack on Pearl Harbor

Backdrop of US-Japan Relations

  • From the early 20th century, US-Japan relations had been relatively friendly, marked by agreements such as the Root-Takahira Agreement (1908). However, Japan's aggressive expansion into China in the 1930s, particularly the invasion of Manchuria, caused tensions to escalate.
  • Japan's imperial ambitions, as envisaged in its Southern Expansion Doctrine, focused on the rich resources of Southeast Asia. This doctrine, however, put it on a collision course with the US, which had its own strategic interests in the region.

Key Developments

  • 1940: The US's response to Japan's growing aggression was to place embargoes on key materials, including aviation petrol and high-grade scrap iron.
  • July 1941: As Japan expanded its territories and occupied French Indochina, the US responded by freezing all Japanese assets and imposing a crippling oil embargo. This put immense pressure on Japan, which was heavily reliant on US oil.
  • September-November 1941: Japan and the US tried diplomatic channels to negotiate, but with significant differences in their stances, little headway was made.
  • 7 December 1941: Taking the Pacific Fleet by surprise, Japan attacked Pearl Harbor.

Reasons Behind the Attack on Pearl Harbor

Strategic Motivations

  • Neutralise Threat: The US Pacific Fleet, stationed at Pearl Harbor, was a significant threat to Japan's expansionist plans. Japan believed a surprise attack would give it a free hand in Asia, at least temporarily.
  • Gaining Time: By disabling the Pacific Fleet, Japan hoped it could quickly consolidate its Asian territories and create a formidable defensive perimeter before the US could mount a counter-offensive.

Political Considerations

  • Show of Strength: Japan wanted to send a clear message of its military prowess and determination to resist Western influence in Asia.
  • Domestic Pressures: Within Japan, the military had a dominant say in policy decisions by the early 1940s. Hardline factions within the military and government viewed a conflict with the US as inevitable and believed a pre-emptive strike was Japan's best option.

Economic Imperatives

  • Acute Resource Needs: With the US embargo, especially the oil embargo, Japan's industrial and military machine faced potential stagnation. The resources of Southeast Asia, especially the oil fields of the Dutch East Indies, became even more crucial for Japan.
  • Desire for Economic Independence: Japan's vision of the Greater East Asia Co-Prosperity Sphere was driven by a desire to establish an economic zone free from Western influence. For this vision to be realised, ensuring unchallenged dominance in the Pacific was essential.

In sum, Japan's decision to attack Pearl Harbor was the result of a convergence of strategic, political, and economic factors, set against a backdrop of international tensions and its own imperial ambitions. While the attack achieved short-term objectives, it set in motion a series of events that would have profound implications for Japan and the world.


Before the attack on Pearl Harbor, several nations condemned Japan's aggressive actions in East Asia. The most notable were Britain and the Netherlands, both of which had colonies in Southeast Asia. Their condemnations usually came in the form of diplomatic protests. Both nations, alongside the US, also participated in economic sanctions against Japan, aiming to cripple its war machine. The League of Nations, particularly in response to the invasion of Manchuria, also condemned Japan's actions, although the League had limited means to enforce any punitive measures against Japan.

The Greater East Asia Co-Prosperity Sphere was an ideological concept promoted by Japan during the 1930s and 1940s. Japan portrayed it as an initiative to liberate Asian nations from Western colonial rule and to promote mutual prosperity, cooperation, and economic integration among Asian nations. However, in reality, it was a cover for Japan's imperial ambitions. Japan sought to create a bloc that would be economically self-sufficient and free from Western influence, with Japan, naturally, at its helm. The idea was to harness the resources of Asia for the benefit of Japan, especially given the economic pressures it faced due to embargoes and sanctions.

The Japanese populace, largely influenced by state-controlled media, celebrated the news of the attack on Pearl Harbor. The successful surprise attack was portrayed as a significant victory and a testament to Japanese military prowess. The media hailed the bravery of the Japanese forces and framed the attack as a justified and necessary action to ensure Japan's rightful place in the world. Nationalistic fervour was at its peak. However, it's worth noting that the general populace was not privy to the strategic complexities and the potential repercussions of going to war with a major global power like the US.

The Pacific Fleet stationed at Pearl Harbor was the primary naval force of the US in the Pacific and thus a direct impediment to Japan's expansionist plans. With its array of battleships, aircraft carriers, and supporting vessels, the fleet had the capability to project power across vast oceanic distances and contest Japanese advances in Southeast Asia and the Pacific islands. Additionally, Pearl Harbor's strategic location allowed the US to rapidly respond to any Japanese actions in the Western Pacific. If Japan wanted a free hand in expanding its territories, neutralising this fleet was seen as an essential preliminary step.

Indeed, there were voices of dissent within Japan's military and political circles against the decision to attack Pearl Harbor. Admiral Isoroku Yamamoto, who would ultimately plan the Pearl Harbor attack, was initially opposed to war with the US. He understood the industrial and technological might of America and doubted Japan's capacity to wage a prolonged conflict against such a power. Likewise, some civilian leaders and diplomats were wary of war with the West and sought diplomatic solutions. However, the hardline military factions that had substantial influence by the early 1940s believed that conflict was inevitable and saw the attack as Japan's best strategic option.

Practice Questions

How did the Tripartite Pact of 1940 influence Japan’s strategic decisions leading up to the attack on Pearl Harbor?

The Tripartite Pact of 1940 was pivotal in shaping Japan's strategic decisions leading up to the Pearl Harbor attack. It bolstered Japan's confidence by providing a semblance of security against potential adversaries, notably the USA and the Soviet Union. Japan felt it could act more assertively in Asia with the support of Germany and Italy. The pact also facilitated stronger economic ties with these Axis powers, offering Japan alternatives to their deteriorating trade relations with the US. Furthermore, by being part of a formidable bloc, Japan hoped to deter the US from intervening in its southward expansion, paving the way for its audacious decision to strike Pearl Harbor.

Examine the economic imperatives behind Japan's decision to attack Pearl Harbor in 1941.

Japan's economic imperatives played a critical role in its decision to attack Pearl Harbor. Following the US's stringent economic sanctions, especially the oil embargo, Japan was faced with the stark reality of resource scarcity. The embargo threatened to stall Japan’s industrial and military operations. Consequently, Japan saw an urgent need to secure resource-rich territories in Southeast Asia, particularly the oil fields of the Dutch East Indies. Furthermore, Japan aimed for economic autarky through the Greater East Asia Co-Prosperity Sphere. To achieve this, dominating the Pacific and stymying potential threats became imperative, driving Japan towards the audacious move at Pearl Harbor.

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Written by: Maddie
Oxford University - BA History

Maddie, an Oxford history graduate, is experienced in creating dynamic educational resources, blending her historical knowledge with her tutoring experience to inspire and educate students.

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