We have different perspectives on this. I think prevention, preventing it from happening. You imply stopping it after it happens. I like to think about nipping these things in the bud. You imply that you deal with it after it has blossomed and is seeded in fertile soil.
But again, I have one line of typing from you to assess your thinking. Thus I leave room for my being inaccurate in what I connote from your one line.
Well, it wasn't prevented because Europe didn't want to risk going to war. It didn't work out that way too.
Now, if something is preventable then yes, prevent it --- but sometimes, things aren't preventable, or what people/countries thought was the safest course of action didn't end up working out. At that point, you have no other choice but to deal with the situation that has arose. That's what happened with WWII. Regardless of whether in hindsight WWII could have been prevented, once Hitler started WWII, the only option was to either sit back and be conquered, or fight him. Did they make the wrong choice by fighting him, according to you?