Now, while the army that can invade the mainline United States doesn't exist today, I'd still say that the insurgent army in waiting is something that we should continue to have. No one is going to invade us this year, or a decade or two, but what about the extreme long term? Fifty years, a hundred, a few hundred? The 2nd Amendment, in the civilian militia standard, is the kind of thing that should be there for the long run.
Are there reasonable rights to civilian ownership of firearms? Damn sure that there are. Is the NRA more than a little crazy? Damn sure they are. But the intercity types and Bradley Campaigners are just as dangerous in my mindset. In an ideal world, I'd like to see more stringent background checks, a registration system that has super super super limited jurisdiction (ideally only on the State level, really. Federal government with my name on a roller-card makes me nervous, no matter how it's organized, I'm afraid), and a greater ability for tax-stamps for some of the more fun toys out there.
Edit for ninjas
Don't worry, an army capable of invading mainland USA would probably have enough artillery for all of them.
Then again, an army capable of invading mainland USA doesn't exist.
Technically, China could get through via attrition, but then again, they'd have to cross the Pacific first.
The Chinese military lacks any kind of force projection, their general equipment and doctrine are not at a high standard, and their training is super spotty.