It could reasonably be argued that both appeasement and isolationism contributed greatly to the outbreak of World War II. As Hitler made ever greater territorial demands in Europe, many politicians believed that giving him what he wanted was a price worth paying for avoiding another world war. The horror and bloodshed of the First World War were still fresh in the minds of millions, and the last thing many wanted to do was to go through yet another destructive conflict. Besides, many countries such as Britain and France were woefully unprepared for war, either economically or militarily.

However, the policy of appeasement failed miserably. It was clear quite early on that once Hitler had been given what he demanded, he'd simply go on demanding even more. And that's precisely what happened after the Munich conference, the high-water mark of appeasement. Though he'd been given the Sudetenland, Hitler had further territorial designs on Europe. Appeasement had emboldened him to go on grabbing more and more land. If he couldn't obtain it by diplomacy, he'd get it by force. And so he did, invading Poland in 1939, which led to the formal start of World War II.

Closely allied to appeasement was the policy of isolationism. This was the dominant attitude of both politicians and public alike in the United States. To a large extent, this was a reaction against America's involvement in World War I. As far as most Americans were concerned, the United States had intervened to make the world safe for democracy, yet Europeans were still at each other's throats. The general consensus was that the rest of the world should sort out its own problems instead of expecting the United States to do it for them. The rejection by the Senate of the League of Nations was an emphatic illustration of the isolationist attitude.

Once again, however, isolationism, like appeasement, helped establish the conditions for the outbreak of war. The United States' lack of active involvement in international affairs had emboldened the likes of Nazi Germany, Fascist Italy, and Imperial Japan, allowing them to carry out acts of aggression against weaker states with total impunity. Without American involvement, the League of Nations was powerless to stop the constant violation of international norms and standards.

Ultimately, isolationism, like appeasement, proved to be a false economy. Not engaging with international affairs meant that serious problems that needed to be dealt with urgently were simply kicked further down the road. So when the United States did finally get involved, it could only be on the basis of another armed conflict, one that might have been avoided had the policy of isolationism not been followed.