Posted by ESC on March 02, 2004
In Reply to: Origin of a phrase posted by Lauri on March 02, 2004
: Where did the phrase "When the balloon goes up" come from?
FROM THE ARCHIVES:
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.
: Observation balloons also used during the American Civil War for spotting (correcting) artillery barrages and detecting troop movement on the battlefield. Any use of the phrase prior to WW1?
balloon goes up, the
The balloon, a colloquial term used of any event, e.g. "What time does the balloon go up?" the speaker meaning, "What time is the parade?"
From _Soldier and Sailor Words_, 1925
what time (or when) does the balloon go up?; with the reply, the balloon goes up at (a stated time). When does it happen? Esp., when does the barrage open or the attack begin: 1915, orig. military; then, 1919 onwards, also civilian.
From _A Dictionary of Catch Phrases, Second Edition_ by Eric Partridge, ed. Paul Beale
The undertaking begins, as in _He's going to announce his candidacy for mayor - the balloon goes up on Monday_. This expression comes from World War I, when British artillery sent up a balloon to notify gunners to open fire, this visual signal being more reliable than courier or telephone. It was soon transferred to signal other kinds of beginning. 
From _The American Heritage® Dictionary of Idioms_ by Christine Ammer