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Japan's invasion of China in 1931 is as good a guess as any.

Yes, but so is the Franco-Prussian war, and the Treaty of Versailles, and the ideas raised in all the other comments. (Okay, maybe not the one about the cow and the lantern.)

Nations choose war if at least one of two conditions apply: They either think they can get away with it, or they think they have nothing to lose.

The defeat of the French in the Franco-Prussian War led the Germans to believe they could get away with war and warlike acts - a perception confirmed when they re-militarized the Rhineland (my own preferred event that 'started' WWII). Japan's invasion of China was another example of an agressor 'getting away with it.'

The German Army never felt they were defeated in WWI. They thought the political leaders lacked strength, brought on in part because the mutiny of the German Navy made them fear the military. So that 'defeat' never hurt their confidence in themselves.

Japan had similar confidence, thinking the self-indulgence of Americans made them weak. A fact - as mentioned - that they confirmed by their invasion of China.

It also takes some desperation when the correlation of forces is no negative, though. The Treaty of Versailles left Germany with nothing to lose, and the economic sanctions directed at the Japanese were the worst possible choice. (A lesson to remember.) It made us look militarily weak, while backing them into a corner where their only choices were attack or die (an economic death).

To return to the point of the original article, Stalin's agreement not to attack Germany was just one more 'we can get away with it factor.' So it was also a 'cause' of WWII.