I've seen this question asked before, but seeing as I can't find it in a search right off the bat, I'll just try to answer as best I can.

I wrote up this post on the Treaty of Versailles, and what it stood for in Hitler's eyes (and how he used it to his advantage). The gist of it is this:

• Hitler used the treaty as justification for his distrust of international powers, and used it to convince other Germans that they had been wronged.

• Hitler used it as a backdrop to demand the unification of Germans, and gaining of space, because the Treaty had crippled them so.

• He also used it as a tool to inflame the Germans, and convinced them that the treaty had intended to make them sub-human when they were not deserving of it (in a way, he was right, but that's all subjective anyways).

But that's just one thing. The Treaty of Versailles was only one of the major factors leading up to the second World War. The argument of the League of Nations, however, is one of the more fanciful assumptions. The League, which never really gained legitimacy anyways, was largely toothless. Much in the way that the UN has failed to stop the crisis in Syria today, and failed to stop the Rwandan genocide, the Bosnian Civil War, etc...the League was ineffective. Even with US support, that wouldn't have done very much. Perhaps it could've discouraged Hitler, but then we get into a dangerous "What if?" game. And besides, if the idea of the UK and France defending Poland didn't stop Hitler from making a non-aggression pact with Russia and attacking, would the US really have been that much more of a deterrent just because they joined the League of Nations, considering that the UK had already pledged to protect Poland, and considering that the US was firmly avoiding the conflict besides to urge negotiations? I don't think so. But that's an opinion, not a fact, and it's debatable. The US more than happily allowed for appeasement, however, in the case of Japan invading Manchuria, and took no actions about that. Joining the League wouldn't have done much, in my eyes, as a result.

However, had the US pledged to protect the Polish and taken steps to reinforce that view, Hitler might've been deterred. Or he could've simply waited it out, and looked for a better opportunity. Again, this is a what-if game.

So what else did lead to the war, besides the policy of appeasement that was mostly advocated for by Neville Chamberlain (really, that just sort of let it happen, we can argue day and night about what would've happened if no appeasement occurred, but we'll never know)? There were a few other things, besides general German aggression.

1. The guarantee of non-aggression from the Soviets. The promise to help divide up Poland, and the idea of the Soviets protecting Germany from fighting a two-front war like it had in World War I, likely encouraged Hitler. That was a big game-changer. Though he did eventually invade the Soviet Union, that's not to say he had much of a choice; Stalin and Hitler were both planning to betray each other according to most of what I've seen/researched anyways. And it's not like invading Russia hasn't worked at all, as some people say; numerous invasions of Russia have been successful, winter or not.

2. The (once again) interconnected web of alliances made it very hard for any nations to resolve things amongst themselves, and led to mobilizations that effectively pushed things to war once anyone started moving. The Poland-France-UK alliance, and the fact that Hitler was out for more land and out for his vision of a German unified world (and a "purer" world) made it a lot easier to have the major European powers (and thereby their colonies) drawn into conflict once again.

3. Hitler's economic policies effectively forced his own country into war. As largely agreed upon by anyone who looks at the economic policies, Germany's focus on military build-up drained it of hard currency, and most of their economic future was unsustainable without war. It is no surprise, then, the the Germans seized gold immediately from the Czechs and Polish. They needed the funds to survive, and their economy was functioning only based on the idea that they could gain more land and more gold by conquering other nations.

That gives a basic idea. I wish I could be more specific; I feel like I rambled a bit much. But the "blame game" is largely a what-if, and much of it (in my eyes) wouldn't have been much of a deterrent to Hitler. Eventually, I think, something would've come up, because Hitler was determined and implacable, and needed more land and gold to keep his economy running while trying to make Europe a little closer to what he wanted it to be. In other words, I think, it was inevitable. Maybe the war could've been less protracted, maybe it could've ended more quickly, but we'll never really know for sure.