The reason for the "misinterpretation" of the second amendment is because of the placement of a comma, in some copies it puts the comma in a place that says only the militia can have guns. In the original document, it says everyone can have them and at any time can organize a militia if the government becomes too tyrannical. The US military oath even says that you only have to obey "lawful orders" (so you can't use the I was ordered to defense when you wipe out a village of civilians with a hacksaw or violate the Constitution.)
Back on topic, it would take an army the size of China's with the transports of the rest of the world (including the US's own) to invade effectively. The invading army would also take massive casualties en route to the invasion point (do you honestly think we don't watch our own shores when we can watch the Iranian president pick his nose on satellite?)
As for industrial capacity, the US has a ton of it still. We just need to open them back up. We also have been hoarding things like oil and metals for the last fifty years (why do you think we import so much while we sit on the world's fifth largest reserve of oil?)
We ARE invadable, but it would take most of the world cooperating to do it. The large militaries don't have the transportation, the one's with the ability to move troops don't have the troops. If the US were invaded, even by China, I don't see the world sitting and watching either. It would be world war three and if the US were about to fall, I have no doubts in my mind that we would launch every nuke we have in retaliation.
