In November 1944 during World War II, the Japanese released 10,000 balloons, made of lacquered paper, filled with hydrogen, and carrying incendiary and fragmentation bombs. The balloons drifted across the Pacific Ocean on the jet stream to land in the Northwestern states of the United States. The goal was to start forest fires. A highly secret "Operation Firefly" was set up to locate and diffuse the bombs and to fight any fires which might start. With a few exceptions, the threat did not materialize.
From the description of United States Army Intelligence records: Japanese war balloons, 1942-1945. (Montana Historical Society Library). WorldCat record id: 610591178