noun
ally; confederate
A person, group, or nation formally cooperating with another for a common purpose. Often used in political, military, or strategic contexts.
彼は長年の同盟者を裏切った。
He betrayed his long-time ally.
その国は戦争で同盟者を支援した。
That country supported its allies in the war.
連合 refers to a union or coalition of groups, while 同盟者 is a person or entity that is part of such an alliance.
Compound of 同盟 (alliance) + 者 (person). The word follows a standard Sino-Japanese pattern for forming agent nouns.