Lucifer:
I was hoping for the short version, but I'll get to both of these books soon I hope.
Thanks Stefan, I'll try this MP3.
I apologize for not getting back to this thread, and you, sooner. Greg's links provide the deepest, most thorough examination of the questions you raise, but quickly, I'll still take a swing. First of all, the "objection" that private defense agencies will turn into warlords has been asked, well, a lot. There must be some primer wherein all the anarchy-won't-work points are raised. ;-)!
Anyway, while I've looked at that issue in essays as well, there's a short one from Bob Murphy that gets at the issue directly, and maybe quicker than you can absorb one of the books. Also, the wonderfully asute and well-spoken Bob Higgs wrote a fantastic essay that spoke to the general issue of why anarchy MUST necessarily be more peaceful than statism. This point I think answers the invasion-by-some-random group question. Also, keep in mind that when everyone is armed, negotiation is much more likely than conflict. Notice that the U.S. generally only invades countries that they KNOW they can beat! In other words, implicit in your question about private defense agencies invading is an assumption that after private defense agencies become common-place, society will eventually be completely unarmed. This is a flawed assumption, since from my point of view (and Stef's, among others) the "fog of war" is a soft requirement for peace.
Anyway, here are some more links:
Bob Murphy's "But Wouldn't Warlords Take Over?"
Bob Higgs's "If Men Were Angels: The Basic Analytics of the State versus Self-government"
"There are none so enslaved as those who falsely believe they are free."
~ Goethe
"Everything the state says is a lie and everything it has, it has stolen."
~ Nietzsche, from "Thus Spake Zarathustra"