noun
countries concerned; parties involved
Used in diplomatic, legal, or international contexts to refer to the nations directly involved in a matter.
紛争の解決には、当事国間の対話が不可欠だ。
Dialogue between the countries concerned is essential for resolving the dispute.
条約の改正には、すべての当事国の同意が
Amending the treaty requires the consent of all parties involved.
Compound of 当事 (tōji, 'the matter at hand') + 国 (koku, 'country').