What is the context of this particular quote?

Adm. Yamamoto always knew that a protracted war with the U.S. would lead to defeat because of the sheer advantage in men and industrial capacity the U.S. had. The US had enough people and factories so that "every blade of grass" could have a rifle behind it and Japan could never match that, but that doesn't necessarily support this idea that he thought Japan shouldn't invade because private citizens already had guns here before the war started. I'll be happy to change my mind if there's any evidence supporting the Second Amendment angle.

I think the idea of an "armed and trained" citizenry has some merit in the case of Switzerland, but again, the Swiss were citizen-soldiers and their deterrance value were as soldiers organized into military units, not as individuals who already had guns.

It's an effective military that deters invaders. On the other hand, it's the gov't and burglars that are afraid of an armed citizenry. Very different situations.