captive balloon

Captive balloons are unpowered aircraft tethered to the ground or a ship by cables. They were extensively used in World War I for observation and artillery spotting. By World War II, their vulnerability led to their use primarily as unmanned antiaircraft barrage balloons, forcing enemy aircraft to fly higher to avoid the cables.1

While manned balloons had limited military use, they were employed for aerial surveillance, such as by the Union Army's Balloon Corps during the American Civil War. Attempts were made to use hot-air balloons burning petroleum for military observations in the early 1900s, but these did not gain traction.23

Balloons, in general, are airtight bags filled with hot air or a lighter-than-air gas like helium or hydrogen to provide buoyancy. A self-propelled, steerable balloon is known as an airship or dirigible.4