countries concerned; parties involved
Used in diplomatic, legal, or international contexts to refer to the nations directly involved in a matter.
Dialogue between the countries concerned is essential for resolving the dispute.
Amending the treaty requires the consent of all parties involved.
当事国 is a country directly involved in a matter, while 第三国 is a country that is not one of the parties.
country involved in a conflict; belligerent state
Compound of 当事 (tōji, 'the matter at hand') + 国 (koku, 'country').