This was a Japanese operation to fly balloons over the Pacific Ocean on the prevailing winds to the west coast of the USA, where time fuses would release explosive and/or incendiary bombs in an attempt to start forest fires and the like (November 1944/March 1945).
The balloons were made of bonded mulberry paper with a diameter of some 91 ft 10 in (28.0 m) and, over its five-month life, the operation launched many thousands of such balloons into the jetstream crossing the North Pacific from Japan toward the west coast of the continental USA, an onboard mechanism venting lifting gas if the balloon climbed too high or dropping a sandbag if the balloon descended too low. Some 200 of the balloons reached Canada and Mexico, while larger numbers reached Alaska, Washington, Oregon and California.
The operation was not successful: only one is known to have started a small woodland fire, while two others killed one woman in Montana and six fishermen in Oregon respectively. The greatest US concern was that the Japanese were seeking to deliver biological weapons, such as anthrax bacteria.