After World War one ended the treaty of Versailles was created, this treaty took away land from Germany, made them pay the cost for the war damages, forced to weaken the military. When Hitler came into power in the early 30's he promised a stronger Germany, one that would overcome the embarrassment of world war one and come out of the depression they were in.
As soon as Hitler was in power he started to violate the treaty of Versailles taking back land and rearming his military. Hitler was able to easily get away with his plans at first because they allies were appeasing him, hoping that if they did nothing the problem would fix itself. But the allies were wrong, every time they let him get away with something he only got stronger, but had they tried to stop him right from the start they could have prevented possibly world war 2 from ever happening. Hitler admitted that had France shown any movement while Germany was invading the Rhineland he would have pulled his troops out right away because he wasn't strong enough to face them at first. Since the allies waited too long Hitler's army grew strong and the war that resulted was hard fought with many deaths that could have all been prevented had a course of action been taken earlier by the allies.
The world certainly learned its lesson from this situation, appeasement doesn't work if you want to resolve a problem you need to take action. You cant just expect to sit on your ass and do nothing and all your problems will magically go away. Appeasement could have been one of the greatest mistakes made in human history considering a whole war could have possibly been prevented had the allies not let Hitler gain strength.
