Would a fortified border stopped the sleeper cells on US soil? (A: no)
Would SDI stop airliners from crashing into buildings? (A: no)
Its time for Libertarians like yourself to enter into the 21st century. 9/11 has demonstrated that a small group of determined, well financed and well supplied individuals can do great harm to us. What kind of defense can the military provide against this modern threat? They can cut off the supplies and the financing by taking out regimes that support terror. They can make it highly undesireable for other countries to associate themselves with terrorists by setting an example.
Iraq is perfect for this goal: they finance terrorists, they can supply terrorists with WMD, and they are easily beatable. A bonus to the defeat of Iraq may be the self-collapse of the current Iranian government. OTOH, North Korea would be more difficult to defeat and diplomatic avenues (which is far cheaper than war) have not been exhausted.
Libertarians need to reconsider the value of pre-emptive strikes as a means of self-defense, and in fact consider pre-emptive strikes as a duty of the government. I believe a Libertarian published such an article several months ago (it was posted on the FR, but I cannot find it).
The war against Al Qaeda should be fought but what in God's name does that have to do with a campaign to conquer and occupy a country run by a secularist, wannabe Mussolini (who spurns Islamic law) have to do with that goal? Your national defense arguments ring allow. This war is being waged for a mixture of Wilsonian goals and regional goals which have nothing to do with home defense and everything to do with spreading ourselves thin.
World policing in Iraq is hopelessly utopian and unrealistic as the sad list of failures in international social engineering demonstrate: Kosovo, Somalia, Gulf War I, Haiti, etc.....but nothing seems to succeed like failure eh?
Instead of being "modern," your proposal takes us back to outmoded policies of the Cold War.