an alliance, union, or league formed by countries or organizations for mutual cooperation
二国は軍事同盟を結んだ。
The two countries formed a military alliance.
企業同盟によって研究開発を共同で行う。
Through a corporate alliance, they conduct joint R&D.
歴史的に複数の国が同盟関係にあった。
Historically, several countries were in alliance relations.