An ancap society would be insanely well armed. It's pretty damn hard to invade a place where literally every person has guns and an individualist mentality. Plus ancaps would still have private militia groups. In the case of an actual invasion I think it's safe to assume that the militia groups would band together as some mutual defense force. The details of who pays for it and contract law between them can be sorted out later. I don't picture an ancap society sitting back and squabbling over contracts while they are being invaded. The obvious huge upside of decentralized defense is that you don't have the risk of the military pursuing it's own interests abroad and hijacking the domestic economy like we have seen in the US. Ancaps would have the peace of mind if knowing that all instances of force are strictly defensive