FIPS 10-4

The 'FIPS 10-4' standard, ''Countries, Dependencies, Areas of Special Sovereignty, and Their Principal Administrative Divisions'', lists two-letter country codes that are used by the U.S. Government for geographical data processing in many publications, such as the CIA World Factbook. The standard is also known as DAFIF 0413 ed 7 Amdt. No. 3 (Nov 2003) and as DIA 65-18 (Defense Intelligence Agency, 1994, "Geopolitical Data Elements and Related Features").
The FIPS 10-4 codes are similar to (but not the same as) the ISO 3166-1 alpha-2 country codes. The standard also includes codes for the top-level subdivision of the countries, similar to but usually incompatible with the ISO 3166-2 standard.