I believe that WWII happened in the first place because of the Triple Entente and their allies deciding that Germany was the ONLY one who started the war, because apparently, nations who start wars in the first place and back out later on can't be blamed for the catastrophes they bring. If they had decided that Austria-Hungary should take some blame regardless of whether or not they backed out of the war or at least lightened the load or waited for the money for repairs to the countries Germany damaged, WWII would never have lead to the suffering and deaths of millions, whether they be Jews, French, German, or even Hungarian. We could've avoided one of the darkest wars in history if the Allies weren't ignorant and selfish enough to ask for money right after a war where Germany spent millions for their military and don't even have any houses or factories anymore, there wouldn't have even been a WWII.
The rise of fascism which caused the European theatre of the second world war was a direct consequence of the closing of the imperial age of the first world war. Hitler's rise to power was very clearly the result - in his own words, even - of Germany's loss of prominence on the world stage after their loss in the first war.
I think that the devastating effect that World War I had on Europe and primarily the economic struggles and restraints that it put on Germany created an environment in which World War II could have come about. The wars were held so close together that there had to be some connection.
The effects of World War I partially caused World War II. Hitler came to power somewhat because Germany was suffering from great poverty after being crushed in WWI. Hitler's powerful speech at first empowered the Germans. When Hitler got to a point of power over the entire country and made allies with other Nazi sympathizers, WWII ensued to break up the fascist stronghold.
The Treaty of Versailles forced the often referred to as proud people of Germany to except blame for the war and the economic hardship of reparations lead to hyperinflation. The German response inevitably lead to a heightened sense of nationalism and an industrial build up of weapons of war and terror.
World War I left the German people beat down and humiliated. This cause a ripple effect when Hitler came into power. By setting the tone and pulling Germany out of a bitter loss, Germany was able to come into great power which caused WWII. This never would have happened if the German people hasn't needed a cause and a leader from previous defeat.