What is the origin of the idiom "the balloon go up" and when to use it?

The idiom "the balloon go up" has its origins in World War I and was used as a code phrase to indicate that an operation or battle was about to begin. The phrase now means that a situation is becoming serious or reaching a crisis point. During World War I, this idiom was used to indicate that a reconnaissance balloon had been released to begin observing enemy positions, signaling the start of an operation or battle. The phrase was used to alert troops that they needed to be prepared for action. Nowadays, the phrase is often used to describe a situation where something serious or unexpected happens, often causing a crisis or an emergency.