In a free country the People Are The Militia. Consider Switzerland. There, every man jack, of sound mind and body, between the ages of 21 and 60, is a member of the militia. As such they are required to keep, in their home, a fully automatic rifle, and hundreds of rounds of ammunition. No one invades Switzerland, and the government doesn't try to take away the rights and privileges of Swiss citizens.
When the Japanese attacked Pearl Harbor, they had no intention of attacking the US mainland, rather they intended to make their moves in Asia on the territories they had acquired and secure an oil supply from Southeast Asia, then sue for peace in international negotiations. They planned to do that, because as their captured government documents showed, that they didn't believe anyone could conquer the USA mainland, because the population there was armed, and too dangerous for any occupying forces.