From Bearing Arms:

The Second Amendment was created with one purpose in mind, and – let’s be honest here – that purpose wasn’t hunting or even self-defense from criminals. While our Founding Fathers supported the right to have arms for those purposes, the primary motivation for the Second Amendment was to protect this country.
That thinking played into the potentially apocryphal comment made by Japanese Admiral Yamamoto, who argued that invading the United States would be a disaster because “there would be a rifle behind every blade of grass.” I say it’s potentially apocryphal because no one has been able to confirm that the admiral actually said this, but it’s the kind of valid observation of America that Yamamoto was known to make.