on active duty
Serving in the military full time, especially being in or prepared to go into battle. My son is in the army, and I worry so much when he's on active duty. After being on active duty for so long, I don't feel fully comfortable being a civilian anymore.
on active duty
in battle or ready to go into battle. (Military.) The soldier was on active duty for ten months. That was a long time to be on active duty.