There are roughly 350 million people in the United States. For the sake of argument, let's say that 150 million are adults, (18+). Now if we say that only half of all households, (75 million), have firearms, that would mean we have the worlds largest standing army. In reality, if a foreign army ever set foot on American soil, it's a safe bet that everyone capable of shooting WOULD be shooting which would make those numbers quite a bit higher. So no, Russia will never invade, or any other country for that matter. The only hope anyone would have of besting us on our own soil would be to nuke us, in which case the US would be so radioactive that you couldn't survive here anyway.