Obsolete abbreviation for Argentina, formed from the first kanji of 亜爾然丁 (アルゼンチン). Used in Meiji-era documents; modern Japanese uses アルゼンチン.
See also: アルゼンチン
On maps from the Meiji era, 'Argentina' is sometimes written as 亜国.
Obsolete abbreviation for the United States, formed from the first kanji of 亜米利加 (アメリカ). Used in Meiji-era texts; modern Japanese uses アメリカ or 米国.
See also: 米国
Depending on the context, 亜国 could also refer to the United States of America.