True civil wars today are started when Generals break away from there commanders. I believe this could very well be the case if ordered to fire on fellow Americans.
What was the intent of the 2nd Amendment? To defend against a tyrannical government. Now think about when the 2nd Amendment was written. The average citizen could own the same weapons as the government and that was it's intent. The 2nd Amendment doesn't need to repealed or weakened, it needs to be expanded to meet today's technologies.
Now do I think every American needs to have the right to go down to Joe's Gun Shop and pick up a Surface to Air Missile? No. But I do think that there needs to be a law that allows, without fear of punishment, any member of the Armed Services the right to refuse orders to attack fellow citizens. That way General's and other high ranking officers could tell there troops to stand down. Then our elected leaders would have no option but to do the will of the people as opposed to there own possibly corrupt agendas.