Japan didn’t so much “get involved” in World War II as it could be considered as having started that massive global conflagration. Hardly the passive harmless nation that emerged out of the ashes of the Second World War, defeated and occupied by the United States Army, Japan was an aggressive imperialist country for many years, even before Adolf Hitler would assume power in Nazi Germany and launch the series of invasions that most Americans consider the starting point of the war. The islands that comprise Japan are largely uninhabitable mountains and rocky hills that were proving inadequate to sustain its growing population and ambitions. Japanese leaders had long eyed with colonial ambitions the vast, sparsely-populated regions of China, especially Manchuria. It had already secured the strategically-important harbor at Port Arthur following its humiliation of the Russian Navy in the 1905 Russo-Japanese War, and was granted territorial rights in Manchuria as part of the Treaty of Portsmouth that officially ended that conflict. The restoration of the Meiji Dynasty in 1868 had brought to power not only the new emperor, but a virulently nationalistic and militaristic cadre of military officers anxious to expand Japan’s territory beyond its immediate shores. Having experienced the humiliations and degradations associated with colonialism themselves, and having observed nearby China’s experiences with European invaders, the Japanese government was determined to industrialize its formerly feudal society and to construct a modern military competitive with those emerging in Europe.
Japanese leaders had been waiting for a pretext to definitively secure all of Manchuria, having invested considerably in developing the region’s infrastructure so as to exploit the region’s natural resources, when a convenient explosion on one of the railways it had built provided the opportunity to carry out a full-scale invasion and occupation in September 1931 that would prove one of the most brutal, atrocious sustained campaigns of human rights violations in modern history. Japan’s occupation of Manchuria set the stage for its eventual invasion and occupation of most of Southeast Asia, excluding only Australia. Its determination to secure natural resources to support its aggressive industrialization, especially the oil fields of Southeast Asia, were the reason the U.S. finally responded by imposing economic sanctions on the Japanese in the summer of 1941. Japan, of course, would respond to the threat it believed the United States posed to Japanese hegemony throughout Asia by executing plans drawn up in 1927 by its Navy to attack the U.S. naval base at Pearl Harbor, Hawaii.
In short, Japan was a principle actor in the chain of events that led the world to its most destructive war.