What role did Japanese imperialism play in igniting World War II?

Japan came late to its status as a modern nation state, saw the Imperial/Colonial late 19th Century model, and aggressively copied it... just as the world was changing again.

After centuries of self-imposed isolation, Japan was pried open in the 1850s and rapidly modernized after that. However, they could sense their vulnerability and observed the behaviour of the Western powers in the late 19th Century. They strove to copy them.

The Japanese arm-wrestled Taiwan (then Formosa) and concessions out of China as peace-terms following a war in 1895, eliminated Russia as a rival in Korea and Manchuria in a war in 1904-05, and snapped up a number of German possessions in the Pacific by coming into WW-1 on the Allied side.

Poor in resources, short on food, and militarily weak, many in Japan believed that an aggressive exploitation of their colonies and possessions and the acquisition of more territory was the way forward. This was compounded by a widespread belief in the racial superiority of the Japanese (again, they were not alone in holding such beliefs at the time).

These pressures and ambitions combined in the creation of Patriotic Societies, which consisted of industrial interests and the military. Moreover, the economic weaknesses of Japan undermined the fragile power of its economic institutions, particularly after the start of the Great Depression. There was also a long tradition in Japanese history of the ambitious governing in the name of the Emperor; disguising their own intentions as Imperial will.

These effects snowballed in 1930, leading to the Manchurian Incident of 1931 and war with China in 1937. Thereafter, the Japanese military and its industrialists were almost out of control and the path to full involvement in the Second World War was set.