It is not true that Japan developed only after WWll. Although quality of life for common people was bad, but Japan was already an industrially developed country long before 2nd WW. It was already an industrial giant that churned out all those war machines with which the Japanese fought simultaneously against China, British, French, Holland and the USA, and at some point with the USSR in the north and SE of Asia. Many of the factories that Japan built in the 1910s in Korean Peninsula are still running.
Even Japan built its first underground Subway line called Ginza Line before that war. It built near about 25,000 km of railway line by 1905 with its own efforts. Comparing to this rly lines in BD is only 2,500 km long, that, too was built during British time.
Anyway, Japan has been so different from other asian nations whose people lack the qualities that the Japanese have. This is why after the war Japan has marched forward to the top of asia as well as the developed world.