When a US amateur is operating from a foreign country under auathorization from that country, you are required to use the frequency allocations permitted by the terms of that authorization. For example, if you are operating in Italy, you are required to follow the Italian frequency allocations.

At times a US amateur may operate using their FCC-issued license in international waters.  When you are operating using an FCC callsign /MM or /AM, you must not exceed the ITU Regional restrictions even if the home country license permits such operation. For example, you must never use the 50 MHz band while cruising in the Mediterranean Sea, conduct SSB operation on 14.110 MHz, etc. However, this does not apply when you are authorized by a foreign administration (except Canada) to sign a portable call such as CP/W1AW. You are only bound by the rules and regulations of the host country.
 

This chart shows frequency allocations for US amateurs operating under FCC jurisdiction but not in US territories and not under the jurisdiction of another country's regulations (such as /Maritime Mobile).

Notes:

Region 1: Africa, Europe, Russia, Middle East (excluding Iran), and Mongolia
Region 2: The Americas, including Hawaii, Johnston I., and Midway I.
Region 3: The rest of Asia and Oceania
(1) Some countries do not give amateurs access to this segment, while others, part of it.
(2) Some countries assign more frequencies around this segment to the amateur service.
(3) In some countries, the segment is narrower than this.
(4) In some countries, the upper end of this segment is lower.
(5) Some countries give amateurs access to this segment by issuing special authorization.