During the Franco-Prussian War (1870-1871), balloons were used for military observation by the armies of both nations. The French statesman Léon Gambetta made a dramatic escape from the besieged city of Paris by balloon.
Armies in World War I (1914-1918) made extensive use of balloons, especially for military observation. The balloons were tethered (controlled by ropes) and used to observe enemy lines. They were quickly lowered when enemy aircraft were sighted. Allied and German pilots received medals for shooting down observation balloons.
During World War II (1939-1945) the barrage balloon (a large fabric balloon tethered to a steel cable) was used extensively to protect London from low-level air attacks. The barrage balloons were tethered in rows at altitudes up to 1,000 m (4,000 ft) to provide a barrier against enemy bombers attacking London. Also during World War II, Japan launched over 9,000 Fu-Go balloons in 1944 and 1945. These were 10-m (33-ft) laminated paper spheres inflated with hydrogen. A timing mechanism periodically dropped ballast (stabilizing heavy weights) to maintain the balloon near the jet stream (the wind that travels from west to east in the upper atmosphere). The balloons carried incendiary bombs and one high-explosive bomb. Only 10 percent of the balloons reached the West Coast of the United States, and most of these landed in the Northwest during the rainy season and caused little fire damage.
Balloons were also used during the Cold War. For a brief period the United States Air Force flew several hundred balloons equipped with cameras over the Soviet Union. A timer controlled the release of the cameras. Once released, the balloons rose rapidly and destroyed themselves in the atmosphere. The cameras descended by parachute over the Pacific Ocean, and U.S. Navy vessels recovered them. After the late 1960s high-flying aircraft and satellites replaced balloons in performing this reconnaissance.