The balloon has gone up

Does anyone know the origin of the phrase, "The balloon has gone up" - meaning the signal to take action has been received? I have a feeling it's war-related but don't know!
Any help appreciated.
christine

When the balloon goes up is a phrase used to imply impending trouble. This relates to the use of observation balloons in the first World War. The sight of such a balloon going up nearly always resulted in a barrage of shells following soon after. The expression was re-inforced during WWII when the hoisting of barrage balloons was part of the preparations for an air raid.