Per IRS Publication 3 Armed Forces’ Tax Guide, page 13:
A combat zone is any area the President of the United States designates by Executive order as an area in which the U.S. Armed Forces are engaging or have engaged in combat. An area usually becomes a combat zone and ceases to be a combat zone on the dates the President designates by Executive order.
To date, the Afghanistan area, the Kosovo area, and the Arabian Peninsula have been designated as combat zones. Combat zone tax benefits have been designated by Congress for the Sinai Peninsula of Egypt under certain circumstances. Each of the combat zones and the Sinai Peninsula are discussed below.