Questions like these can appear to make sense on the surface if you make certain assumptions about the ancient world. If you believe it was a world of very small self-sufficient communities in a state of perpetual undeclared war, maybe it would make sense for each of these communities to enslave anyone they met and didn't know. But it pays to think a little harder about whether such a world could really exist. If people could expect to be captured and sold into slavery by any strangers they met, how would you build ties with other communities? How would you trade or forge marriages or make alliances? How would you conduct any business at all, whether public or private, if the world outside your own little village was a hostile anarchy?

To put it simply: people travelled without fear of being randomly enslaved because it was in everyone's interest to make sure that did not happen. The very survival of each city depended on their ability to travel and communicate safely.

Admittedly we don't have much evidence of treaties on travel safety between communities like the cities of Ancient Greece, but we know there were unspoken agreements between them that travellers, envoys and merchants were not free game. I've written previously about the divine sanctions against killing messengers, which appear to have been as common to the Persians as they were to the Greeks. Other forms of travellers - pilgrims to sanctuaries, athletes and spectators to panhellenic festivals - were similarly protected. Major festivals like the Olympic Games came with a period of ekecheiria, literally "hands-off," in which even states that were in open war against each other were not allowed to touch travellers through protected lands without incurring the wrath of gods and men. Even objects might be included in these exemptions. In 373 BC, the Athenian general Iphikrates invoked a curse on his city when he captured and sold Syracusan offerings on the way to Olympia and Delphi. The Syracusans may have been enemies, but the gifts (and the envoys carrying them) belonged to the god.

Of course, this is not to say that there are no examples of the arbitrary enslavement of people who happened to be at the wrong place at the wrong time. Border raids were a pretty timeless constant in Greek history, and the most desirable plunder was cattle and humans. Naval raiding, too, was endemic, and piracy was a scourge to merchant shipping and coastal communities. In that sense it is true that merely existing in the ancient world carried a risk of being enslaved. But - and this really shouldn't need stressing - the community that had its people stolen wasn't about to stand by and let it happen. Carrying off the inhabitants of a state to sell them into slavery was an act of war.

In 491 BC, when the Aiginetans captured a ship full of Athenians on their way to a festival at Sounion, it rekindled and ancient feud that would only end two generations later when the Athenians annihilated the last settlement of Aiginetan refugees. There were many reasons for the outbreak of the Peloponnesian War in 431 BC, but one popular version told at Athens was that the war started because some Athenian youth had kidnapped a Megarian girl, and the Megarians had kidnapped a pair of sex workers in retaliation, with things escalating rapidly from there. (The comedian Aristophanes joked that if a Spartan stole a puppy, the Athenians would launch 300 warships to avenge it.)

Festivals in which women travelled outside the city were moments of particular anxiety about enslaver raids, and this could have disastrous consequences. In the aftermath of the Greek defeat against the Persians at Lade in 494 BC, the surviving warriors from Chios retreated into the territory of Ephesos - but the Ephesians mistook them for an invading army out to capture their women, and slaughtered them to a man. There were no regrets or apologies about this. If there were no courts or councils to appeal to, the last resort was to defend the people with weapons in hand.

These may be extreme examples, but the point is that there were, in fact, tacit codes of conduct between states, and these were invoked to justify policy and backed up with force if necessary. You couldn't simply seize a stranger and expect to get away with it. People didn't live in splendid autarky and isolation; they were part of communities that had ruling bodies and laws and armies, and these communities saw themselves as part of wider networks that had obligations toward each other to behave according to the standards that benefitted everyone. In such an environment it was generally safer to let strangers be.