The Japanese sent balloons over to set fires here and there during WWII.
...and nobody knew about them until ten years after the war. They didn’t cause any noticeable damage.
There were some claims that the Japanese WWII balloons were for biological warfare but .gov kept it secret.
That fact was kept secret to prevent panic. In total, about 9,300 balloons were launched, of which about 300 were discovered in North America. On May 5, 1945, six civilians were killed near Bly, Oregon, when they discovered one of the balloon bombs in Fremont National Forest, becoming the only fatalities from enemy action in the continental U.S. during the war.
The most tactically successful attack took place on March 10, 1945, when one of the balloons descended near Toppenish, Washington, and caused a short circuit in power lines supplying the Manhattan Project's production facility at the Hanford Engineer Works. Backup devices quickly restored power at the site, but it took three days for its nuclear reactors to be brought to full capacity; plutonium produced in the reactors was later used in Fat Man, the bomb dropped on Nagasaki in August 1945.