Why is the world so different now than it was in the first half of the 20th century?

I've read tens of dozens of books about the early 1900's, especially WWI and WII. I amazed to step into that old world, and then walk out into modern society to see such stark differences. For example, it is amazing to me that in the 1930's, Germany had political inations almost every week and that was seen as normal. Then the hyperinflation in Germany... things like that. Of course we've had our 9-11 and things of that nature, but overall you walk outside and the world seems pretty stable. What changed? And in your opinion, do you think it can only remain this way for so long before there's some other huge conflict that comes out of the blue like WWI did?