A diplomatic term for a country with which one's own nation maintains friendly relations. Often used in political or international contexts.
The two countries have cooperated as friendly nations for many years.
友好国 is a more common and slightly less formal synonym for 'friendly nation'. 友誼国家 is more literary and emphasizes the bond of friendship (友誼).
Compound of 友誼 (friendship, friendly relations) and 国家 (nation, state). The word 友誼 itself is a Sino-Japanese term combining 友 (friend) and 誼 (friendship, goodwill).