The above answer discusses at some length whether one person can influence history to such an extent as World War II. It should be noted that more words have been written about Adolf Hitler than any other individual in history, with the sole exception of Jesus Christ. It is true that the circumstances in Germany were volatile; but it should be remembered that the Weimar Republic had managed to maintain order and stability in Germany for several years after the war, even in the face of communist agitation by Carl Liebnecht and Rosa Luxemburg, who were determined to bring down that government. It should be noted too that Hitler's first attempt to take over militarily, the Beer Hall Putch, failed miserably.
One cannot discount the personal charisma and magnetism which Hitler brought to Germany and the German people. The National Socialist German Workers Party was already in existence before Hitler became famous; he came to it first as a spy for the army to keep an eye on it. While attending his first meeting, Hitler got caught up in the moment and delivered a diabtibe which endeared him to the members who soon made him their leader. It was his gift as a speaker and his ability to plan military strategy alone (although that strategy was ultimately unsuccessful) that led Germany into the war. Additionally, it was the publication of Mein Kampf which made him a national hero. It is quite possible that the war would have happened anyway; yet Hitler was the right man and the right place with the right gifts. It may not be true that Hitler was the sine qua non of the war; but he most assuredly was the direct and efficient cause of its outbreak in Europe.
Historians will differ vigorously over this question for more than one reason.
First, there is the issue of whether individual human beings really have much of an impact on history at all. Some historians argue that it is really larger forces like economics and geopolitics that cause things to happen, not individuals. Similarly, some scholars of international relations argue that nations act in their own interests, regardless of who their leader is. In this view, WWII was not caused by Hitler but rather by Germany's desire to gain power and the Allies' desire to prevent Germany from doing so.
Second, even if you allow for the idea that individuals can impact history, you could still argue that Hitler wasn't the main cause of the war. You could argue that the real cause of the war was the Treaty of Versailles. In this view, the war was caused not by Hitler's desires but by the fact that the Treaty of Versailles was so harsh to Germany that it made Germans much more likely to go to war again to escape from the treaty's provisions.
My own opinion is that the war would likely have happened even without Hitler. Germany was likely to want to push the French and British very hard over the Treaty and those two countries were likely to fear Germany no matter who its leader was. It could be, however, that Hitler made the war come faster than it might have because he was so aggressive in his attempts to get Germany back to what he saw as its rightful place among nations.