[Wellsy]

What has been the historical tendency in dealing with remaining enemies when one group wins control?

Like, how was the USSR purge? What happened to monarchists and supporters of the British after the US war of independence?
How did France react after the nazi’s were kicked out? How did ancient civilizations treat enemies?

And after stating such an account, what is your judgement about the efficacy of
those actions in retaining control?

I keep speculating that even in a modern government it makes sense to remove those with opposing ideological commitments from pivotal positions. At the same time there is presented a dichotomy however of pursuing ideological purity over competence and promoting useful idiots.

When it comes to an entire society I wonder what one makes of say the French whose police collaborated with the Nazis and institutionally continue a legacy based in that subordination.

And I wonder how things might’ve been different in regards to the American civil war where there seem policy decisions that are incredibly lenient on traitors and wonder what the trade off is in being lenient if not risking the perpetuation of an opposing ideology.
But then without resorting to some extreme mass slaughter of a people, there are effective attacks on political organization of certain ideas that may persist but never fond substantial ground and are broken again and again across generations.

[wat0n]

I would not say the southern states got a lenient treatment. They were downgraded into territories under a federal military government and forced to adopt constitutions consistent with the US one. Those who didn't like it would sometimes riot - often targeting Black freedmen - and the federal government would repress them using the army.

The rehabilitation of a southern former state would depend on how quick was it in adopting a Constitution following federal guidelines and the most stubborn ones would only be admitted again into the union in 1875 or so IIRC i.e. as part of the Reconstruction Era. Traitors who violated their oath to the US Constitution were barred from holding any public office in the US and any state government unless 2/3 of each house said otherwise (14th Amendment).

Of course, they weren't treated as harshly as they could have been but they were treated more harshly than, say, Germany after WWII. Yet as you suggest there are limits to that if you don't want to enslave or physically destroy the defeated - which wasn't all that rare some centuries ago.