To drive a vehicle legally in Japan, you will need an international driving permit (IDP) etc. The nationality (which country your passport is issued) will not affect the the validity of the license or permit necessary to drive in Japan. Japan and America is bound by the U.S.-Japan Administrative Agreement, in which US military personel and their families are eligible to drive with the driver's license permitied in the agreement aside from the international driver's permit. For details, please see the US page.