On p. 10 of the book, I wrote: “I am painfully aware that there are many issues of importance I will not have touched on in the book. You will probably find the phrase ‘But what about…?’ forming in your head from time to time as you read. I can only plead in extenuation that in a book of such modest proportions I have had to be extremely selective in my choice of topics.” Since practical issues have been treated by such as Friedman (and, of course, Rothbard) and others, I concentrated more or less completely on the theoretical issues.
The problem you raise is a very important one and one that David Friedman makes a valiant effort to address. It is something I am thinking about at the moment, if somewhat obliquely, in the context of my next book project, which will concern itself with the relationship between freedom and authority.
In brief, however, here is what I think.
In a condition of terminal anarchy where there are no states, there will be no specific problem of national defence. However, unless and until that condition is reached, there will be a problem of how individuals or groups of individuals can defend themselves against aggression by remnant states
In anarchy, it will most likely be the case that defence against individual aggression will be provided by specialist agencies. This much is standard. However, not only would it be likely that defence agencies would have cooperative arrangements for the solution of individual disputes, it is not inconceivable that they could offer as a service the organisation of defence against group aggression, utilising effectively a militia strategy. By this I mean that in the extraordinary situation where a whole area or a whole group of communities is under attack, the people themselves would function as a defence force. Having your protection agencies provide the officering would then allow such a force to be effectively mobilised.
This idea is not pure speculation. It is, more or less, the historical situation that the Swiss cantons found themselves, surrounded as they were by the Austrian Empire, France, and some German states. The Swiss army is organised for purely defensive purposes. Every Swiss man of an appropriate age has a duty to be prepared to defend the whole and the organisational structure is light. The Swiss strategy might be described as ‘the hedgehog’, an animal that is unpleasant to eat and spiky and it has been remarkably successful.
These remarks are cursory and I’m sure that you will have no problem thinking up scenarios that might prove difficult to deal with. However, Getting from here (statism) to there (anarchism) throws up a whole raft of transition problems. My inclination at the moment is to think that a Proudhonian federalist solution of gradual dissolution is probably the best practical way forward but I haven’t fully thought these matters through and my views might change as I do more research.