Their stand on open boarders would only work if the Feds weren't in the habit of giving a free ride to every puddle-jumper who can make it across the boarder. This has been clearly stated a number of times. You just can't seem to get past the idea can you? All you see is "open boarders", and it pretty much shuts down any thought on how it would work.
As for military defense. Don't be absurd. If we are attacked, then we have every right to fight back. We don't, however, have a right to attack another country. Iraq has attacked us. Afghanistan has attacked us. Hell, the case could be made that China and Britain have both attacked us (spy plane and the UK's mechanations in the UN).
Ask yourself, would 9-11 have happened if the Libertarians view of the Second Amendment been the norm?