Empires fail because they stop considering people as important. They get sloppy and end up creating the force that overthrows them through their disregard of life. When you start talking about "acceptable casualties" you've already lost the mental war. You're seeing people are abstract numbers.
The key to winning a conflict against empire and the key to not becoming yet another empire is cultivating a core trait of empathy that respects life above all else and rejects violence as anything but a tool for self defense and liberation. It doesn't teach it only harms. We need radical compassion