To drive in many areas of operation [ other than Mexico ], one needs to procure an international drivers license. International Driving Permits are Required* in the following countries for US Licensed Drivers: Austria, Bosnia-Herzegovina, Croatia, the Czech Republic, Germany, Greece, Hungary, Italy, Poland, Romania, Slovenia, Slovakia, and Spain
Now why would you think those countries require an international license and not accept your USA drivers license?
Unless things have changed in recent years, which I doubt, an International Driver's License is nothing more than a piece of paper. It provides your local driver's license information in an internationally standard format and in alternate languages so that someone in let's say Spain can more readily determine what your foreign license status was when you applied for the piece of paper. There are (or were) no competency tests required to obtain an international driver's license that might make you more qualified to drive as a tourist in their country.
I will leave it to others to chime in if any of the above is materially wrong.