the balloon went up

As the film, Argo, about the escape of US hostages from the besieged US Embassy in revolutionary Iran contends for an Oscar as Best Picture, a recent book on the Iranian Revolution describes how in late 1978 Britain’s Ambassador, Sir Anthony Parsons, watched violent, pre-revolutionary events aimed at overthrowing the Shah break out across the country. He reported to London that “the balloon went up”, a common expression at the time for a major conflagration. The author explains that it “arises from WW1 when British artillerymen sent up balloons to signal to their own trenches the imminent start of firing.”

Days of God by James Buchan, published by John Murray, 2012