Population of Japan in 1945. 71,998,104 . Troops stationed in Japan 350,000. 1 Solider for every 205 people.

Population of Afghanistan somewhere between 30 to 34 million. (you are not going to get an exact number for there hasn't been a census in Afghanistan since 1979.)
There are currently 133,148 NATO troops in Afghanistan. So 1 Solider for every 225 or 255 people (depends on which estimate you use).

We have spent several magnitudes of money greater than Afghanistan GDP on the Afghanistan War.

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Now we used to have far less troops in Afghanistan during the first few years of the Afghanistan war. This was a mistake. At the same time it is a massive mistake to compare the US wars of Japan and Germany in the 40s to the current war today.

The culture is completely different
Afghanistan has a completely different history, it does not have an educated population, its literacy rate is low.
There was no organized opposition in Japan or Germany in 1945. In Afghanistan we let some of the old institution escape and once they escaped they hide and performed guerrilla war on us since we removed them from power.
There have been studies that it is hard to form democratic institutions in stressful situations, due to poverty and constant fighting peoples brains shut down and they enter a state of depression. Much of Afghanistan is suffering a form of post traumatic stress disorder since the country is a shithole.

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We didn't lost the Afghanistan rebuilding due to money and solider commitment. We lost it because we thought that was all we needed. It wasn't effort what we needed, it was fighting smarter not harder.