I don't know... too many imagine how can anarchism replace our huge countries and their governments. An anarchist society is a little larger than a tribe. The question stops being how do you defend from the United States of America and rather how do you defend this anarchist neighbourhood. 

 Anarchist societies could be quite large (think city-states with millions of residents or large rural counties) and private defense is a cornerstone of the anarchist structure. Private defense contractors are very effective, much more than police today. 

 Because they can loose the contract if they abuse the citizenry or abdicate their duty?  

 When you don’t do well at your job you don’t get paid and you don’t get repeat clients. Private security has been a very successful model globally for millennia.