Rare photographs of military observation balloons of World War I, 1914-1918

Observation was an incredibly important role for aerial warfare in World War I. All major combatants used observation balloons to observe their enemies' trench lines and troop movements.

These hovering mammoths were used to direct artillery, which required a spotter and observation beyond the visual range of ground-based observers.

As much as aircraft were able to record enemy positions and motion on film, it was necessary to have real-time spotters and observational balloon baskets connected to the ground by telephone. This allowed artillery to take advantage of increasingly large guns with very long ranges.

The first military use of observation balloons was during the Battle of Fleurs (1794) by the French Aerostatic Corps during the French Revolutionary Wars.

The oldest preserved observation balloon, L'Intrépide, is on display at the Vienna Museum. They were also used by both sides during the American Civil War (1861–65) and continued in use during the Franco-Prussian War (1870–71).

World War I was the high point for the military use of observation balloons. Despite their experience in Africa in the late 1800s, the British were behind the development and were still using spherical balloons.

These were quickly replaced by more advanced types, known as kite balloons, which were shaped to be aerodynamically stable and could operate in more extreme weather conditions. The Germans first developed the Parseval-Siegsfeld type balloon, and the French soon responded with the Caquot type.

Typically, the balloons were tied to a steel cable attached to a winch, which would reel the gasbag to its desired height (often above 3,000 feet) and retrieve it at the end of an observation session.

Because of their importance as observation platforms, the balloons were defended by anti-aircraft artillery, groups of machine guns for low altitude defense, and patrol fighters. Attacking the balloon was a risky undertaking but some pilots preferred the challenge.

The most successful were known as the Balloon Busters, which included notables such as Belgium's Willy Koppens, Germany's Friedrich Ritter von Roth, America's Frank Luc, and the Frenchman Leon Bourged, Michel Coeford and Maurice Boyau.

Many specialist balloon busters were careful not to go below 1,000 feet (300 m) to avoid contact with anti-aircraft guns and machine guns.

The phrase "the balloon is going up!" As an expression for impending battle it is derived from the fact that the ascent of an observation balloon potentially indicates an initial bombardment for an offensive.