[dragonwriter]

> A captive population is part of what creates an incentive to seek power through violence.

I'd say it is almost the reverse; the desire to establish a captive population is what leads one to seek power through violence.

> If people have freedom of movement, it's much harder to subjugate them by holding land.

If you hold land by force, you can thereby deny people freedom of movement in any direction. That's actually the only way to deny people freedom of movement, whether you concentrate on the inbound direction (as many countries do whether or not they also care about the other direction) or the outbound direction (Cold War East Germany, for a well-known example).

[TomMarius]

The problem isn't that they were held (ISIS didn't have resources for such large scale operation), but that no place has accepted them.

[dragonwriter]

“No place accepted them” is also largely denial by force (both at boundaries and in the interior of they manage to evade it at boundaries), just from the other direction.