This is probably false. Yamamoto's "rifle behind every blade of Grass" quote is probably a myth. Likely the Japanese simply never had the capability to invade the US to begin with.
A funny story from the WWII: During the 'blitz' Britain enlisted the help of the NRA to get American gun owners to send them weapons and ammunition. Britain had enacted strict gun laws in the 1930s and as a result the home guard didn't have enough guns to go around, much less resist a German invasion.