It wasn’t that much money. It invested about $18 billion inflation adjusted dollars into Japan from 1946 to 1952. By contrast it has invested $145 billion into Afghanistan, a country with half the population Japan had back then.

This comparison probably doesn't make a lot of sense: the US's reconstruction of Japan is backlit by Japan being an industrialized nation with an educated workforce, in a country with basically no domestic conflict in the century prior to WW2. Afghanistan was and is not industrialized to nearly the same degree, has nowhere near the same degree of educational penetration, and has a long history of domestic conflict in the forms of colonialism and sectarian violence.

In other words: $1 in 1945 Japan goes much further in terms of development than $1 in 2000s Afghanistan.

Fair enough, but in a way it's more false comparisons; investing billions into a tribal-agrarian society in the midst of civil and religious war is qualitatively different from investing it into an already industrialized society. And it's not like the absolute $$ investment was the sum total of what the US was doing in Germany and Japan. It came along with a whole package of political-economic moves to restructure investment and regulation, taking advantage of American hegemony in those regions.

> Fair enough, but in a way it's more false comparisons; investing billions into a tribal-agrarian society in the midst of civil and religious war is qualitatively different from investing it into an already industrialized society.

Not necessarily. If you ignore the (in reality rather important) "ongoing war" aspect, investing in more backward region should give you better return on investment, not worse. That's because investment, like anything else, suffers from diminishing returns. In backward regions, there are a lot of low-hanging fruits to pick, and small amount of money can go very long way. That's why all the NGOs are talking about microlending etc: even trivial amounts of capital make a huge difference.

Low floor, low ceiling: microlending can set someone up to put their children through school, but it can't turn subsistence farmers into lawmakers and engineers (at least not overnight).