Pretty widespread. There were, however, plenty of people who rolled their eyes at this belief, even before Hitler came to power and thoroughly quelled that notion.

It wasn't just that countries thought that the level of destruction and death that resulted from World War I would convince nations to abstain from war, governments actively pressed for treaties and social changes that would lead to global disarmament.

The most major of these diplomatic advancements was, of course, the League of Nations. The League of Nations was primarily seen as a forum where nation's could settle disputes that would have, in earlier times, led to war. The League actually saw modest success here! It's major flaws were just that it was too weak, it relied on members stepping up and doing the right thing, which didn't always happen.

But one of the League's greatest (and most futile) goals was global disarmament. And, of course, they couldn't achieve this, for obvious reasons. Countries tend to be suspicious of one-another and announcing disarmament could be seen, by a particularly war-like country (cough, Germany, cough), as an invitation to invade.

The Kellogg-Briand Pact was another example of well-intentioned pinheadery. This pact, signed by most of the world, essentially outlawed war. It remains in effect, and it's longterm success is worth debating, but in the immediate aftermath of it's signing, it failed tremendously.

And finally, the policy of appeasement towards Hitler was a part of this belief that war after the Great War was something that must and could be avoided.

Few actually thought war was entirely finished after the First World War. The thinking was a war of that magnitude would never happen again because of an increased use of diplomacy (and the advancement of technology made it easier for opposing nations to communicate).