noun
participating nation; participant country
Refers to a country that takes part in an international event, treaty, conference, or competition.
オリンピックの参加国は200以上にのぼる。
The number of participating nations in the Olympics exceeds 200.
この条約の参加国は年々増えている。
The number of countries participating in this treaty is increasing year by year.
加盟国 means 'member state' of an organization, implying formal membership, whereas 参加国 simply means a country that participates in a specific event or agreement.
Compound of 参加 (participation) + 国 (country).