'36 for the Spanish Civil War, the "opening act." '37 for Japan in China. And when did Italy try invade Ethiopia, '35?
But I can see the logic of tying it to 1 Sep '39, the invasion of Poland, which resulted in most of Europe being drawn in on one side or the other.
Fun WWII fact: At the beginning of the war, most combatants still had fabric-covered biplanes in active operational service. Just six years later, two combatants had operational jets, with a third on verge of having them operational. That is a quantum leap of technology in that amount of time.