I’m not sure when WWII started in Europe, but the only reason I think the US had such a boom during the late 30s – 70s is because of the industrialization of the world and everyone else who had manufacturing capacity was dead/bombed out already.
The US came out mostly unscathed after WW2 so they were ripe to dominate the world economic stage when the war ended.
Unless WWIII comes, I don’t forsee us repeating a great middle class economy since there really is no need for the slack labor out there. Liberty/freedom, self-reliance and rugged individualism doesn’t amount to a whole lot if you can’t find decent work/job because there is limited opportunity. You can see it already with recent college grads who are not of the STEM flavor. After 5-10 years working in a non-degree needed job, they are just going to fall further and further behind.
One reason I believe luck has a lot more to do with one’s financial success a lot of times since being at the right/place time trumps hard work.
