"Submission" can be tolerable or harsh. "Tolerable" is whatever the population will accept without open rebellion. Guerrilla activity is open rebellion at some level. The Second Boer War was rather brutal compared to the First, which Britain lost. They weren't going to lose again. There was ongoing guerrilla activity, but Britain dealt with it and retained its conquests until after WWII.If You occupy a place and want to keep it on long term/indefinitely/for ever, making population to accept submission means You're already close to failing, if not already starting failing.
Edit: The OP puts the poster in the position of waging war, not making policy.