noun
allied nation; ally
General term for a country in a formal alliance, especially in military or political contexts.
日本とアメリカは同盟国です。
Japan and the United States are allied nations.
その条約により、両国は同盟国となった。
Through that treaty, the two countries became allies.
noun
Central Powers (WWI)
Historical term referring specifically to the Central Powers in World War I (Germany, Austria-Hungary, etc.).
第一次世界大戦では、ドイツやオーストリアが同盟国側だった。
In World War I, Germany and Austria were on the side of the Central Powers.