"The Balloon Went Up."

You see this phrase as the opening of hostilities, as in “If the balloon goes up, I’ll go go from reserve status to active.” What’s the origin of that? What balloon, where?

It originated in WWI. Balloons were raised above the line preceding an artillery attack. (Sources vary whether the balloons were raised as a signal to commence firing, or as observation balloons.)

A news paper report from 1900, http://trove.nla.gov.au/newspaper/article/85825310

Boer war… "There is a battle’ going on to-day 16’ miles from
here. We can see the balloons, but tho wind is,
in the wrong direction to hear the guns. The
sentries on the heights say they can see the
smoke of the big guns. A train has just come
in and reports that tho British have got 8,010
Boers hemmed in. "

Note that the french used balloons in 1790’s and the Mongols used unmanned , small, signal balloons as far back as the last 100’s, ( circa 200 AD ) ,eg when the Han dynasty chancellor Zhuge Liang (Kongming) was surrounded by Wei dynasty General Sima Yi at Pinlo, Sichuan.

But the expression may well be Civil war related, where the state militia were raised when the war came to the state. (Those militia having a constitutional right to bear arms…)

Why would the state militia have been called a balloon?

The Union used balloons for reconnaissance during the Civil War, but their use was very limited. Interestingly, the Google Ngram viewer shows a small peak for “balloon goes up” around 1860, but the phrase didn’t become common until after 1920, making a WWI origin more probable.

The balloon barrage was much used in England in WW2 to deter aircraft from built-up areas. A mass hoisting of barrage balloons took place in September 1939 when the war started.