constitution that was essentially
written by Americans
I cannot speak about Japan, but that's historically false for Germany. If you look at the list of participants in the main events that drafted the West German constitution such as the Rittersturz-Konferenz [1] and the
Verfassungskonvent auf Herrenchiemsee [2], then you'll see that all participants are German. Naturally, the allies had to agree, but they were split in a big way. The Soviet Union anyway, but the US and France really had very different ideas. France, in particular, wanted to snatch the Saar region. Naturally the drafters of the German constitution were inspired in particular by the US constitution, because it was rather successful.[1] https://de.wikipedia.org/wiki/Rittersturz-Konferenz
[2] https://de.wikipedia.org/wiki/Verfassungskonvent_auf_Herrenc...