Because enforcement under anarcho capitalism is non territorial, defense and law enforcement are indistinguishable except for the size of the criminal gangs that one must defend against, no difference between street gang and a government.
Since one is defending people, not places, there is no free rider problem. An individual free rider runs a risk that he may have difficulty enforcing law and custom when a crime is committed against him. He relies on the kindness of strangers for the safety of his property, his liberty, and his life.
Similarly for a group, for example a militia, which attempts to free ride. Large scale threats endanger it.
Anarcho Capitalism can be criticized that its solution to the free rider problem (death) is excessively drastic, but you cannot criticize anarcho capitalism that it will be overrun by free riders. Anarcho Syndicalism will be overrun by free riders, forcing the syndicalists to use statist and eventually totalitarian measures, as happened in the end in Catalonia, but not anarcho capitalism.
It is likely that an Anarcho Capitalist defense against a large scale threat would be inefficient, costly, destructive, and bloody, compared to the defense mounted by a more centrally organized form of social order. This is however a different issue to the free rider problem, though closely related.
An anarcho capitalist defense against a large scale threat would very likely resemble the defense of America against the British during the revolutionary war. The British could go where they pleased and destroy whatever they wished, but were unable to obtain political control by so doing, and no matter where they went, and no matter what they did, people kept killing them from time to time..
Anarcho-capitalists would find it very hard to organize a static large scale territorial defense based on barbed wire entanglements, trenches, and minefields, and if they needed to do such a thing, they would have to temporarily give up anarcho capitalism, but this form of warfare has been the exception rather than the rule. The objective in warfare has usually been to kill or intimidate the enemy, rather than to take or defend a particular hill, and anarcho capitalism will probably do well at that task. Although static warfare was dominant during World War One, this may well have been a result of mass conscript armies of reluctant, unskilled, poorly equipped cannon fodder, rather than an inherent requirement of sound military tactics and the military technology of the day.
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