Some very personal opinions from my part...
Japan lost millions of people, almost all of its cities of more than 50,000 people were severely destroyed, the second most powerful fleet was sunk, and Japan was occupied (I think it was the only time in its history), the use of the bomb (some versions say merely to "impress" Stalin), and currently one of the most important strongholds of US power is
Okinawa...
You hardly can be more defeated than that.
(However, sometimes some severe destruction of physical infraestructure could be beneficial, since Japan was forced (like Germany) to replace that infraestructure using the most advanced standards - compare with England).
The initial ocupation with McArthur verified political reform, although the initial US plan was to keep Japan "low"... a position similar with the intention of applying the Morgenthau Plan.
However, seeing the evolution in Eastern Europe, the beginings of the Cold War, the advance of Communism in China, convinced the Americans to let loose, that a weak Japan was a risky thing.
I do not agree that it was "American investment" what produced the material advance of Japan. Rether, it was the acceptance of the Americans to allow the reconstruction of the conglomerates (Zeibatsu, now Keiretsu), of a more dynamic Japanese capitalism. I remember that much of the ammo the Americans used in Korea, was produced in Japan.
Later, the Japanese, with an original "manufacturing ideology", that included things now famous in manufacturing worldwide like SCQ (a system developed by an American, Deming), Kanban, Just in Time, Reverse Engineering... and the education of many Japanese abroad, with the idea of seeing the most advanced techniques in any area (something that the Chinese are doing today), and with strong State interventionism ("Japan Inc.", the MITI, etc)... the Japanese where the ones that did the job.
In my view, it was not that the USA did much FDI in Japan (on the contrary, the Japanese put a lot of barriers to US capital in Japan, since they wanted to develop its own renewed and original capitalism). Although certainly, the USA import of Japanese products, sustained that development until at least the 80s.
Currently, Japan is the 2nd Economy of the World, although its influence around the World ouside the mere economic sphere, is much lesser that the USA today.
Just in the case of a severe economic problem in the USA (maybe due to an irresponsible financial management) we could see Japan as a real number 1 in the forseeable future.
Regards.