debt to one's country; national obligation
Formal, somewhat literary term for the moral debt or gratitude one owes to one's nation. Often used in patriotic or historical contexts.
彼は国恩に報いるために軍に入った。
He joined the military to repay his debt to his country.
Compound of 国 (country) and 恩 (grace, obligation). The exact historical derivation is uncertain, but the term appears in classical Chinese and was adopted into Japanese with the same meaning.