The Japanese called the balloons fusen bakudan. Thirty-three feet in diameter, they were made of paper and were equipped with incendiary devices or high explosives. In less than a year, nine thousand were launched from a beach on Honshu. They killed six people in Oregon, five of them children, and they started forest fires, and they landed from Alaska to Mexico and as far east as fifteen miles from the center of Detroit.
During World War II the Japanese used balloons as weapons? Really? Wild! They even killed people with them. On the U.S. mainland. No surprise that "papers were asked not to print news of them." Scary enough to listen for bombers buzzing in the sky, but a silent, slowly drifting attacker that could fall upon and destroy you purely at random? Sounds like — Terror! In a New Yorker article John McPhee says the government wanted the story kept quiet so the Japanese wouldn't find out about their success.
And I have to say the fire balloon campaign's strangest success — or near miss, rather — would be the balloon that caught "on a high-tension line carrying power to the reactor" at the secret Hanford installation, "the reactor that was producing the Nagasaki plutonium." The reactor was temporarily shut down.
source: "Checkpoints" by John McPhee, The New Yorker, February 9 & 16, 2009