Grassy Knoll — Poul Anderson

The benevolent, paternalistic World State regarded the freedom-minded Jeffersonians as a minor embarrassment whose violent elimination would cause more disruption than their demise would merit. So both sides were happy when the chance came for voluntary exile to a distant planet. But two hundred years later the less benevolent descendant of the World State that had let them go was to decide that the cosmos was not big enough to hold both it and a free people.