I never knew Japan attacked the U.S. with balloons during WWII until I did the search. Japan started the suicide pilot attack also.
Who needs an ICBM when you can launch an air balloon with a nuke in the Pacific and let it float to the target. Japan was successful at exploding conventional ordinance in the U.S. delivered via ballon from Japan, according to the article.
From Fire Balloons
I thought you might find the link in reply#8 interesting.