noun
partner country; other country; opponent country
Used in diplomatic, trade, or competitive contexts to refer to the country one is dealing with, negotiating with, or competing against. The exact nuance depends on context: it can be neutral (partner), contrastive (the other side), or adversarial (opponent).
貿易交渉で相手国との合意を目指す。
We aim for an agreement with the partner country in trade negotiations.
サッカーの試合で相手国に勝った。
We beat the opponent country in the soccer match.
自国 means one's own country, often used in contrast with 相手国.
Compound of 相手 (partner, opponent) and 国 (country).