What is "National Defense"?
If you mean simple defense of private property against would be attackers, you would accomplish this the same as you would in defending your home from a robber. Through voluntary individual action, either through contract with an agency of your choice, or through voluntary cooperation (i.e. militias).
If you mean some type of standing military, you completely miss the concept of individual liberty and that of the nation state. For starters, an 'invader' must have something to gain for the expense of his invasion. In a 'nation state' which commands a monopoly over a terrirory and it's resources, the gain is obvious, to either usurp or greatly influence that monopoly over the resources. In a completely decentralized situation, where INDIVIDUALS or at most voluntary communities control their fratctional and justly acquired portion of resources with no overarching monopolistic territorial control aperatus, there is simply no gain. Say that Invader A was able to take over block A1 of Washington D.C. in a decentralized scenario. Great, good for them. They have now gained block A1's resources. However, the other 99.99999999999999% of the decentralized territory remain out of their control. It is a scenario likeable to infinite energy, wherein energy output received will ALWAYS be less than energy input. Likewise, in a completely decentralized scenario there is simply no payoff, the investment in 'invasion' will always be greater than any return that could be realized.
And of course, if you are of the belief that "national defense" requires pre-emptively intervening in the affairs of another region to increase your interests, you are the invader and might as well be casting your vote for Rick 'Nuke em All' Santorum.