The accepted cause is Hitler. That is palpable nonsense. Even if he had never been born, that war would have occurred and at about the same time ie twenty years after the end of the First World War, when Germany had time to recover and rearm.
In 1918 Germany asked for an armistice. The United States Commander, General Pershing, warned President Wilson not to grant one so that Allied forces could invade Germany and make it population taste defeat. The general predicted correctly that, if such an invasion did not take place, Germans would believe wrongly that they could have won the war by fighting on and therefore eventually wage a war of revenge. Wilson ignored Pershing's advice and concluded the Armistice in time for the American November 1918 elections.
The stage was thus set. Firstly there was popular outrage throughout Germany. Of special concern was the loss of eastern territory inhabited by a million Germans (including the Polish Corridor). Politicians of all German Parties (including the Communists) called it the "Bleeding Frontier" and promised to rectify it. All inter-war German Governments, including Socialist ones, rearmed in defiance of the Treaty of Versailles.
The people yearned for a revanchist leader. In 1926, the Swiss psychiatrist Carl Jung undertook a tour of duty of German mental hospitals. On his return to Switzerland, he predicted the rise of a dictator in Germany within ten years because that is what the German collective subconscious was demanding.
The Great Depression provided the trigger for open revanchism. General Schleicher became Chancellor in 1932 and espoused it openly. He was, however, outmanoeuvered and replaced by Hitler in 1933, but the Second World War would have ensued anyway.