The morality of the state.

In Junior High, I read The Moon is a Harsh Mistress by Heinlein. In it Heinlein asks, through the character of Prof, "when is it morally permissible for the state to commit an act that would be immoral for an individual?" (I paraphrase.) Over the years, I have tried to draw that line in my mind, but so far have failed to do so to my own satisfaction. Sometimes I feel the answer should be "never", but that doesn't really work in the real world. So where is the line?