In the migration crisis, the EU is currently acting just like the ancients, as if border controls did not exist, though the mass, peaceful migration we see today was not a tremendously common occurrence then.

The reason is that in the ancient world, every male was a potential warrior. So in conflict they would either fight to defend their land and, if they lost, be killed or sold into slavery, or they would flee, en masse, as Germanic tribes did into the Roman Empire in the 4th century ad, escaping the Hun onslaught.