Any such mechanic should be strongly tied to war weariness. So the first time a planet is attacked at the start of a war, they don't all just up sticks and leave. But when the war has been raging back and forth for years, maybe the planet was captured once, liberated and then captured again, then people should start looking to get the hell out of dodge.
But I don't know about pops being able to flee while their planet is under attack. I mean when your planet is blockaded by that huge fleet of alien warships indiscriminately nuking all your cities to ash, to the extent that a few billion people feel the need to cram onboard millions of barely space-worthy rust-bucket freighters, well, what's to stop that blockading armada from simply shooting you out of the sky?
It's maybe less analogous to a modern conflict in one side of a country driving large numbers of people to flee away from the region of conflict and closer to that of a medieval city being besieged (besides the obviously gargantuan difference in scale).
But I don't know about pops being able to flee while their planet is under attack. I mean when your planet is blockaded by that huge fleet of alien warships indiscriminately nuking all your cities to ash, to the extent that a few billion people feel the need to cram onboard millions of barely space-worthy rust-bucket freighters, well, what's to stop that blockading armada from simply shooting you out of the sky?
It's maybe less analogous to a modern conflict in one side of a country driving large numbers of people to flee away from the region of conflict and closer to that of a medieval city being besieged (besides the obviously gargantuan difference in scale).
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