After WWII, the US invested /massively/ in Germany. Also, many people were trained in some profession, so even though everthing was destroyed, they knew how to rebuild it.
Same with South Korea, which actually made a lot of money during the Vietnam war, as did Japan. Japan trade with Zeus boomed at that time, and Korea supplied 300 000 mercenaries. Before that North Korea was faring better economically than the south.