World War I was at the time and in the years just after described as the "war to end all wars". The phrase was in part a reaction to the horrors of the conflict with many believing that with the full cost of modern war so evident no nation would fight one again. It was partially an outgrowth of the original reasons for going to war seeming insignificant as the conflict dragged on, and the notion was adopted as an explanation for why continued sacrifice was needed. The notion the conflict being a war to end all wars was also closely linked to the portrayal of Germany among the Allies, the country was seen as the heir of Prussian militarism, and a longtime warmonger. To end warfare the defeat and destruction of this militaristic power in the centre of Europe was a necessity.
After the war a number of initiatives were taken to try to assert the promise. This included the formation of the League of Nations, the rewriting of the map of Europe, the Kellogg-Briand Pact outlawing war, and a number of other treaties limiting military power.
With the outbreak of World War II the phrase quickly began to be seen ironically. However WWII, at least in Europe, would in many ways be the war to end all wars with centuries of conflict between the great powers having not resumed since 1945. The end of world spanning conflicts was brought about as much by nuclear weapons and Mutually Assured Destruction as idealism, however.
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