Imperial Japan had their own rodent army they plan to use against the US. They wanted to release plague infected insects into the population where they would first attack rodents stateside. They would be released via clay pots carried by balloons. It was to be an attack on the West Coast of the continental US
They also had these, which some people have thought may have been what came down at Roswell; the remains of one was found as recently as 2014:
https://en.wikipedia.org/wiki/Fire_balloon