The failure to build up an industrial base within its regions of control, as well as the inhuman brutality of the Imperial Japanese Army are what ultimately led to the fall of Imperial Japan. 

Japan is an industrialized power with a low quantity of natural resources to sustain a war, namely in fuel, steel, and rubber. Its materials of war were acquired by either buying from resource rich countries such as the United States, or through the exploitation of its colonies in the puppet state of Manchuria and plantations in Southeast Asia.

However, Japan's brutal methods (such as during the Rape of Nanjing) means that it was never able to quell the rebellions within its territory of control nor effectively uses their natural resources, and said methods also led to the sanctioning by the United States government from exporting more resources to Japan. So, Japan was losing the war well before their attack on Pearl Harbor, and the entry of the United States into WWII only greatly hastened its defeat. 

It is important to note that fascism, such as the kind practiced by Imperial Japan, is ultimately unsustainable, as it will always run out of resources to fuel the never-ending expansion of conquest, which is why it could be said that self-destructive nature of fascism guaranteed the fall of the Empire of Japan.
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