volunteer soldier
A soldier who enlists voluntarily, as opposed to being conscripted. Used in military contexts.
彼は志願兵として軍隊に入った。
He joined the army as a volunteer soldier.
志願兵 is a soldier who joined voluntarily, the opposite of someone taken in through 徴兵, compulsory conscription.
Compound of 志願 (volunteer, application) and 兵 (soldier).