After becoming aware of the jet stream over the Pacific Ocean that blows toward the United States and Canada, some high-profile military scientists came up with the the brilliant idea of strapping some bombs to balloons.
The idea was that the balloons would inconspicuously float over the ocean, haphazardly drop fire-bombs on the western world, then celebrate their balloon-fueled victory. To the people on the receiving end of the death balloons, the Japanese must have looked pretty desperate.
However, World War 2 was the first and last time the world will ever see the infamous war balloons. Japan launched almost 10,000 gently floating bombs, and only about 300 were observed in North America. As Japan figured out that day, there is more than just one direction of wind, and a huge chunk of those balloons just lazily floated around Japan, presumably causing Japan to re-evaluate the credentials of its military scientists.
Of the 300 balloons seen in North America, 6 people died and some buildings were hurt. According to math, these balloons had an astonishing kill-percentage of 0.0006%. Which we are pretty sure makes fire-balloons about as lethal as birthday balloons.