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Why did the balloon bombs launched by japan, failed as effective weapons against america?

They were ineffective weapons because:

• The major reason for it's ineffectiveness lay in the fact that they had a very low success rate. They killed only six people and caused only a small amount of damage which amounted to a kill rate of 0.067%.

• The expense required in the preparation of these balloons was large.

• The preparations were lengthy because the technological problems were acute. A hydrogen balloon expands when warmed by the sunlight and rises, then it contracts when cooled at night, and falls.

• America always knew that japan had been working on biological weapons and therefore somewhat anticipated such an attack.