What Motivated Japanese Aggression in World War II?

Japanese soldiers advance in 1940
Keystone, Hulton Archive / Getty Images

In the 1930s and 1940s, Japan seemed intent on colonizing all of Asia. It seized vast swathes of land and numerous islands; Korea was already under its control, but it added Manchuria, coastal China, the Philippines, Vietnam, Cambodia, Laos, Burma, Singapore, Thailand, New Guinea, Brunei, Taiwan, and Malaya (now Malaysia). Japanese attacks even reached to Australia in the south, the U.S. territory of Hawaii in the east, the Aleutian Islands of Alaska in the north, and as far west as British India in the Kohima campaign. What motivated a formerly reclusive island nation to go on such a rampage? 

Major Factors

Three major interrelated factors contributed to Japan's aggression during and in the lead-up to World War II. These factors were:

  1. Fear of outside aggression
  2. Growing Japanese nationalism
  3. Need for natural resources

Japan's fear of outside aggression stemmed in large part from its experience with western imperial powers, beginning with the arrival of Commodore Matthew Perry and an American naval squadron in Tokyo Bay in 1853. Faced with overwhelming force and superior military technology, the Tokugawa shogun had no option but to capitulate and sign an unequal treaty with the U.S. The Japanese government was also painfully aware that China, hitherto the great power in East Asia, had just been humiliated by Britain in the first Opium War. The shogun and his advisers were desperate to escape a similar fate.

After the Meiji Restoration

To avoid being swallowed up by the imperial powers, Japan reformed its entire political system in the Meiji Restoration, modernized its armed forces and industry, and began to act like the European powers. As a group of scholars wrote in the 1937 government-commissioned pamphlet, "Fundamentals of our National Policy": "Our present mission is to build a new Japanese culture by adopting and sublimating Western cultures with our national polity as the basis and to contribute spontaneously to the advancement of world culture." 

Changes Had Wide-Ranging Effect

These changes affected everything from fashion to international relations. Not only did Japanese people adopt western clothing and haircuts, but Japan demanded and received a slice of the Chinese pie when the former eastern superpower was divided into spheres of influence at the end of the nineteenth century. The Japanese Empire's triumphs in the First Sino-Japanese War (1894 to 1895) and the Russo-Japanese War (1904 to 1905) marked its debut as a true world power. Like the other world powers of that era, Japan took both wars as opportunities to seize land. Just a few decades after the seismic shock of Commodore Perry's appearance in Tokyo Bay, Japan was on its way to building a true empire of its own. It epitomized the phrase "the best defense is a good offense."

Growing Importance and Influence

A sometimes virulent nationalism began to develop in the public discourse as Japan achieved increased economic output, military success against larger powers like China and Russia, and a new importance on the world stage. A belief emerged among some intellectuals and many military leaders that the Japanese people were racially or ethnically superior to other peoples. Many nationalists emphasized that the Japanese were descended from Shinto gods and that the Japanese emperors were direct descendants of Amaterasu, the Sun Goddess. As historian Kurakichi Shiratori, one of the imperial tutors, put it, "Nothing in the world compares to the divine nature of the imperial house and likewise the majesty of our national polity. Here is one great reason for Japan's superiority." With such a genealogy, of course, it was only natural that Japan should rule the rest of Asia.

Rise of Nationalism

This ultra-nationalism arose in Japan at the same time that similar movements were taking hold in the recently unified European nations of Italy and Germany, where they would develop into Fascism and Nazism. Each of these three countries felt threatened by the established imperial powers of Europe, and each responded with assertions of its own people's inherent superiority. When World War II broke out, Japan, Germany, and Italy would ally themselves as the Axis Powers. Each would also act ruthlessly against what it considered to be lesser peoples.

Not All Were Ulta-Nationalists

That is not to say that all Japanese were ultra-nationalist or racist, by any means. However, many politicians, and especially army officers, were ultra-nationalist. They often couched their intentions toward other Asian countries in Confucianist language, stating that Japan had a duty to rule the rest of Asia, as an "elder brother" should rule over "younger brothers." They promised to end European colonialism in Asia or to "liberate East Asia from white invasion and oppression," as John Dower phrased it in "War Without Mercy." In the event, the Japanese occupation and the crushing expense of World War II did hasten the end of European colonialism in Asia; however, Japanese rule would prove anything but brotherly.

Marco Polo Bridge Incident

Speaking of war expenses, once Japan staged the Marco Polo Bridge Incident and started its full-scale invasion of China, it began to run short of many vital war materials including oil, rubber, iron, and even sisal for rope-making. As the Second Sino-Japanese War dragged on, Japan was able to conquer coastal China, but both the Nationalist and Communist armies of China put up an unexpectedly effective defense of the vast interior. To make matters worse, Japan's aggression against China prompted western countries to embargo key supplies and the Japanese archipelago is not rich in mineral resources. 


In order to sustain its war effort in China, Japan needed to annex territories that produced oil, iron for steelmaking, rubber, etc. The nearest producers of all of those goods were in Southeast Asia, which—conveniently enough—was colonized at the time by the British, French, and Dutch. Once World War II in Europe erupted in 1940 and Japan allied itself with the Germans, it had justification for seizing enemy colonies. In order to ensure that the U.S. would not interfere with Japan's lightning-fast "Southern Expansion"—in which it simultaneously struck the Philippines, Hong Kong, Singapore, and Malaya—Japan decided to wipe out the U.S. Pacific Fleet at Pearl Harbor. It attacked each of the targets on Dec. 7, 1941 on the American side of the International Date Line, which was Dec. 8 in East Asia.

Seized Oil Fields

The Imperial Japanese armed forces seized oil fields in Indonesia and Malaya. Those countries, along with Burma, supplied iron ore, and with Thailand supplied rubber. In other conquered territories, the Japanese requisitioned rice and other food supplies, sometimes stripping local farmers of every last grain. 

Became Overextended

However, this vast expansion left Japan overextended. Military leaders also underestimated how quickly and fiercely the United States would react to the Pearl Harbor attack. In the end, Japan's fear of outside aggressors, malignant nationalism, and demand for natural resources to support resulting wars of conquest led to its Aug. 1945 downfall.

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Szczepanski, Kallie. "What Motivated Japanese Aggression in World War II?" ThoughtCo, Mar. 14, 2021, thoughtco.com/japanese-aggression-in-world-war-ii-195806. Szczepanski, Kallie. (2021, March 14). What Motivated Japanese Aggression in World War II? Retrieved from https://www.thoughtco.com/japanese-aggression-in-world-war-ii-195806 Szczepanski, Kallie. "What Motivated Japanese Aggression in World War II?" ThoughtCo. https://www.thoughtco.com/japanese-aggression-in-world-war-ii-195806 (accessed March 26, 2023).