Given history, even with League of Nations, WWII would have happened, here is why... The world did not want another war. This is why the international world let Japan annex Korea, attack China, Philippines, Taiwan, Pacific Islands, and so many countries in Southeast Asia. The Japanese if not stopped, would have continued to grow, and the international community continued to try to appease them. Hitler took over Austria, the Sutenland, broke the Versailles Treaty by building its Army, yet countries didn't want a war, so try to appease Hitler as long as they stopped.
Nevermind that the Jewish people and other minorities were already being killed, and Hitler's Aryan Third Reich wanted more "living space." They, like the Japanese, believed they were superior in race. Japan had annexed Korea, an independent nation (killing, raping, torturing its citizens) and the world turned the other cheek. This was in 1910, before WWI and the Treaty of Versailles. Japan was well under way in trying to expand its country at the expense of others.
Hitler and Japan believed and successfully for decades called the international community's bluff. Finally, it became a tipping point and by the time the world opened up their eyes, they say that Japan and Germany's true intention were not for peace, but to get rid of anyone that stood in their way. What is most dangerous was it was based on purely ethnic lines. Koreans were forcibly taken to Japan and severely discriminated against. Japanese could kills Koreans at anytime without repercussions. The same for the Nazis against Jews, Slavs, (anyone not Aryan). Even if the international community continued to try to appease Japan and Germany, the countries that were conquered would have eventually continued to try to revolt, inevitably creating a situation where all-out war is impossible.