The fact that Japan and Germany were the major parties in WWII, means that they were powerful industrial nations, with large and educated populace. Even though WWII devastated Japan and Germany more than most other Great Powers such as USA, UK or even France, they still had a decent platform to build their nation up again.

One shouldn't also forget that after the war, both Germany and Japan stayed strongly within the American sphere of influence. USA was interested in helping to build up Europe and Japan to stop the spread of communism. Japan also benefitted from the Korean war, where it was the major supplier of UN forces, giving a boost to Japanese economy.

Both Germany and Japan have been at peace after WWII, have had stable economic policies and been members of international trade. They were strong nations before the WWII and they are still today.

Mancur Olson, a political economist, addresses this in his book The Rise and Decline of Nations

The argument is this: Germany and Japan, by being major combatants in the war, demonstrated their industrial capacity and relatively advanced economic development. The participation in the war was the result of an accumulation of special interest groups that swayed policy.

The war left the commercial capacity--the human capital and the networks of trade--mostly intact. But it destroyed the special interest groups that had gained power. Olson argues that economic growth is usually restricted by special interest groups who want to prevent the disruption of their current industries. This combination, the disruption of special interest groups and the negative relationship between special interest groups and economic growth, leads to his conclusion that the war, paradoxically, allowed Germany and Japan to grow faster than they would otherwise.

Obviously a very simple answer, and the details are a lot more complicated than just this. However, it is a model that fits the story you're asking about.

Another thing people seem to be missing here is that compared to a lot of other economies, especially say Britain, Germany and Japan had the advantage, not only of educated and well-trained people, but of no war debt. Unlike, say Britain, which almost bankrupted itself during the Second World War, Germany and Japan could build afresh and anew, while also receiving huge amounts of aid from the United States. Finally, any obsolescent and old industry that might have caused backlash among workers and held back technological development when it started to decline, such as in Britain's industries, Germany and Japan could build only what was then in demand, rather than saddling the burden of older industrial development. An interesting book on this is Postwar: A History of Europe since 1945, by Tony Judt.

The case of Japanese : The Japanese felt a devastating economic loss after the war, though we have to realize that, the US only managed to attack several cities in Japan, which include Tokyo, Nagasaki, and Hiroshima. Though these cities that mention are destroyed. But the infrastructure outside these cities, especially the rail tracks, are relatively still intact. This make the movement of goods can be back to normal on a short period of time. Furthermore, one thing that i notice is that post the occupancy of the allied forces, Japan is joining the Allied forces as a major supply hub for the Korean War. This position give a boost in Japanese economy, although limited. The biggest part is actually post Korean War when Japan conducting a special economic revitalization program, which named "The National Double Income" This proposal is introduce by PM Hayao Ikeda, this program rely upon analysis of strength and weaknesses of Industrial Sectors in Japan. The analysis they used is by using the Input Output model by Leontief. Through this analysis Japan choose Heavy Chemical and Machinery as their main industry that need to be developed. After the proposal started to be implemented, Japan economic growth grow faster and become one of the developed countries. Source : Economic ideology and Japanese industrial policy : developmentalism from 1931 to 1965 by Bai Gao

It's no coincidence that the most powerful countries with the strongest economies which went to war against each other in WWII still have some of the strongest economies today. US, China, Japan, Germany, Russia, UK, France, Italy, is this a list of WWII belligerents? Or is it a list of the 10 largest economies of the world today minus Brazil and India (which was part of the British Empire during WWII of course)? It's both.

I think there was a social component as well. That is in both defeated countries all the old power structure was destroyed, and the people left in charge had few ties to the past. A lot of the Japanese emphasis on quality was a reaction to seeing the quality of Allied equipment first hand, and then asking for help, which they got in the form of Edward Demming and Joe Juran in the 40's and 50's - two world class quality problem solvers who both learned from the famous Walter Shewhart in the 20's at AT&T. The Japanese listened and put what they heard into action, the result was Toyota and Honda, Nikon and Sony. Back in the US, these guys were ignored until the 80's.

I got here late, but a key element economically speaking was that they both had high savings rates during and after the war. Their GDP % grew faster than the U.S. in the decade following WWII.

Do you think that not being allowed a standing army means more money and emphasis on the economy?

After their defeat, both quickly became front-lines against the what was seen as a Soviet Union bound on world conquest. Neither of these countries could be 'lost' to the SU -and neither really had to pay much for their own defense during this crucial period. No amount of economic, political, or military spending was too small to support these two countries.

Let's not forget that Germany and Japan also had a load of help rebuilding (allies in West Germany and Berlin, USSR in East and USA in much of Japan too).

Compare this with Britain, who not only had to rebuild, mostly independently, with loans to help them on the way though, and a fracturing empire. It wasn't until the mid to late '50s that rationing ceased in the UK, and with India etc. gone, they were essentially buggered.

No military comes to mind