An interesting question. A more question more to the point would be: Can there NOT be a public monopoly on national defense? Who would pay for fighter aircraft, tanks, and nuclear missiles in a anarchist society (assuming an anarchist society can exist)? After all, since the threat of invasion is collective for a community, most individuals wouldn't see the benefit to themselves, so nobody would want to pay for it. This phenomenon is known as the Free Rider Problem (http://en.wikipedia.org/wiki/Free_rider_problem). Even if some people did pay, what of those who do not pay?

The monopoly of the USPS on letter mail is much different. Letter delivery is a desirable service to the recipient and the recipient alone. It is far easier for a mail truck to not deliver letters to non-paying customers than for a military to refuse to defend territory owned by those who did not pay into it. The two aren't the same situation, so try to stay on topic.