When travelling I plan / expect to present my passport to demonstrate that I have the proper standing (paperwork) to pass through customs and immigration at border control points. While abroad, I carry my US Passport card, which is issued by the US department of state:
The passport book and passport card are both U.S. passports. If issued for the full validity, they are both proof of your U.S. citizenship and identity.
Are there any nations that will not recognize the passport card as a form of Identification? I was stopped in Paris by a policeman and was asked for a passport and I left it in the hotel safe: I offered to show it to him as the hotel was 100 meters down the street, but he was not interested. It would be nice if I could present the passport card to confirm my standing to be in the country (Americans do not need Visas in the EU), however, it is the passport book that confirms standing.
As I understand it, an American driver's license is not acceptable in Europe for the purpose of identification.