One way of determining whether Western democratic intervention in WWII was necessary is to compare a hypothetical “What if they hadn’t intervened?” postwar map against the postwar map which actually pertained.
Even if the Western democracies hadn’t intervened, war would still have taken place. The Chinese and Japanese would have fought each other in China, and Germany and the Soviet Union would still have gone to war.
Had the Western democracies chosen not to involve themselves or participate, France and Britain would have retained their independence. Everything east of France would have been owned by some combination of Axis nations and the Soviet Union.
In the east, the Chinese would eventually have thrown Japan out of their country. (Though it might have been necessary for them to have gotten their civil war sorted out before that happened.) The real question in the Pacific theater is whether Japan would have attempted to grab British, French, or Dutch colonies in the Pacific theater. Had such an effort been made and had it been successful, Japan would have ended the war with more colonial territory; and the European nations with less. The question would then have become: would Japan have been able to maintain control over its possessions over the long run? Or would those colonies have eventually achieved independence? (As they eventually did under European colonial rule.)
