Was Hitler solely responsible for World War II?
Hitler was perhaps more responsible for World War II than any other individual, but he was not solely responsible for it.
First, we must remember that World War II had both a Pacific theater and a European theater. Hitler certainly did not cause Japan to invade China or to attack Pearl Harbor. So we cannot blame him for the war in the Pacific.
While Hitler is much more responsible for the war in Europe, he is not solely responsible. We can put some blame on the people who created the Treaty of Versailles. That treaty gave Germans some reason to be very angry after WWI. It gave them a real reason to want revenge. This desire for revenge helped lead to WWII. We can also, perhaps, blame the French and British leadership during the early 1930s. When Hitler was first rebuilding German strength, either of these countries could have stopped him if they had been willing to do so. Hitler was not yet strong enough to cause a war early on. If France and/or Britain had stopped him from reoccupying the Rhineland, for example, he would likely have lost credibility in Germany and the war might never have happened.
Hitler, then, bears the greatest responsibility for the war, but he is not solely responsible for it.
No single person can be responsible for something as complex as a war. It requires many thousands of people to make armies, fund them, provide logistical support, etc. That is while the "great man" (or "evil man") way of accounting for historical events is generally more a feature of popular culture than historical scholarship.