Oomis, World War I broke out on Tisha B’av. Although the holocaust happened in WW II, they say that aschuli dpuranus adif. (A phrase which means the beginning of the tragedy is the main day. This is why we observe Tisha B’av on the ninth, and not the tenth. On the ninth at the end of the day, the fire was lit, but the actually destruction occurred on the tenth. This is also why we do not shave or eat meat until noon on the tenth.)
WW1 set the stage for WW2, because the Germans were embarassingly defeated, and forced to disarm and abide by various humiliating sanctions imposed by the Treaty of Versailles. This caused Hitler to steam and stew, and he looked for a scapegoat. He invented the concept that it was not the Germans that lost, but rather that the Jews in Germany had betrayed Germany and stabbed them in the back. It was an internal collapse. WW2 was to get “revenge” on the Jews for this defeat.