This is the 8590th most frequent Japanese word.
援軍
Reinforcements or allied troops.
In this context, '援軍' refers to military reinforcements dispatched to support one's forces.
敵の攻撃に備えて援軍を送りました。
We sent reinforcements to prepare for the enemy's attack.
Here, '援軍' denotes the arriving backup forces that change the tide of a conflict.
援軍が到着して、形勢が逆転した。
The reinforcements arrived, and the situation was reversed.
In this sentence, '援軍' represents the reinforcements or help the soldiers anticipate for support in the battlefield.