Can you understand the war without it? Probably not. I'd say you roughly need a good understanding of European politics in the interwar period, especially the chip on Germany's shoulder over Versailles, to really get to the root of Hitler's Nazism movement.

That said, it is not necessarily the case that WWII should be seen simply as a byproduct of WWI and Versailles or a continuation of the first war - the story is more complicated than drawing a straight line of causation would suggest. It's obviously important - un-excludable, even - but it's not the only factor that played into Nazism and the coming of WWII.