Peace is a goal, and not a system. If your system for achieving "peace" is a police state, and you find that your "adversaries" are always able to overcome or work around them, eventually you may realize that the police state (tyranny, authoritarianism, etc.) is ineffective in creating a stable peaceful state.

That doesn't mean that achieving a peaceful state as a goal is bad, or necessarily impossible, but that the method/system applied to achieve it was not effective.