As to guns in private hands detering invasions. The Japanese disagreed with you. During WWII, the Japanese were considering a raid on the Northwest US, in order to force the US to divert resources to defend this area. They decided against it precisely because they knew how well armed and trained the citizens were. This is not theory or conjecture, the minutes of the meetings were captured when Japan surrendered. During WWII, civilian patrols were quite influential in protecting British shores against sabuetors.
[This link now appears to be dead but if you do a search for some of the phrases you can still find the text on the web.–Joe]