How could World War 2 have been prevented?

During the 1930s, Hitler acted belligerently, violating the treaty of Versailles and seizing various pieces of territory. Britain and France, rather than opposing him at the risk of war, kept giving him what he wanted in the hopes of appeasing him. If they had ever stood up to him and gone to war if necessary (while Germany was still weak), they could have stopped him once and for all. Instead they waited until Germany was too strong to beat easily, and World War II was the result. I will say, however, that it is my opinion that even if Hitler had been stopped in the 1930s, there still would have been a second world war sometime in the 1940s against Stalin's Russia. Stalin was just as interested in world conquest as Hitler was; Hitler just beat him to the punch.

Essentially, to stop World War II, all the allies needed to do was stand up to Hitler and say no. If they had stopped the re-occupation of the Rhineland, or the Anschluss of Austria, or the Sudetenland crises, the German generals would have deposed of Hitler. If the Allies had objected strongly enough with the threat of military action against the trashing of the Versailles treaty, Hitler would have been stopped. Any of these actions would have led to the probable loss of face by Hitler and possibly see him ousted by the generals. This would have led to a potential coup and a Germany led by the military, which may still have involved World War II happening. The root causes for WWII lay way back in WWI and the Versailles treaty. If this treaty had been not so harsh, or if the Allies had indeed wanted to help Germany instead of blaming her, then the underlying causes for World War II might have been avoided.