Economic integration was the main difference between the end of WWI and WWII, and we know how those turned out. One resulted in a horribly burdened and depressed German people. When a charismatic figure who could and would speak to the people emerged (Hitler) preaching nationalistic righteous anger, he rose to power to horrific results for both Germany and the rest of the world. It could have happened to any country in the same boat.

After WWII, the allies learned from their mistakes and instead of saddling the German people with mountains of debt and economic punishment, they sought to rebuild, quickly established trade and economic integration. It went substantially better the second time, and to this day seems to be working.