noun
allied forces; allied armies
Refers to military forces that are allied together, often in a formal coalition or treaty.
同盟軍が敵の要塞を包囲した。
The allied forces surrounded the enemy fortress.
連合軍 also means 'allied forces' but often implies a looser or more temporary coalition, while 同盟軍 suggests a formal alliance.
Compound of 同盟 (alliance) + 軍 (army). The exact historical derivation is uncertain, but the meaning is transparent from its components.