Postng to his extremely thoughtful, interesting new blog, UCLA professor Mark Kleiman formulates, to my surprise, what he refers to as the "one-free-bite rule" of international relations: while "preventive" warfare to topple a dangerous-looking regime is unjust, a government (he has Iraq in mind) that has already engaged in sufficiently aggressive behavior (invading a neighbor, for instance) forfeits its moral immunity from preventive attack. I mention my surprise becase I have been advocating for several years my own slightly different, more "realist" (more cynical, some might say) version of the "one-free-bite rule". As I see it, a run-of-the-mill nasty dictatorship can pretty much be expected to engage in "useful hostilities" with at least one of its neighbors, so as to distract the masses, justify its own iron grip on power, and keep its military too preoccupied to hang around the capital planning coups d'etat. But a regime that attacks a second neighbor--and particularly one that scores successive victories in each--can be assumed to have ambitions beyond its station, and thus to require removal for the sake of regional stability.
To use my original example, a Slobodan Milosevic "merely" ravaging Bosnia could be (and probably was, by many) considered to be an ordinary thug exploiting nationalism to shore up his internal support, and hence only offending the world's conscience (however appallingly). By following up with the strongarming of Kosovo, on the other hand, he made himself look more like a dangerous megalomaniac who would keep using his army until stopped by vigorous application of NATO military force.
Likewise, Saddam Hussein was tacitly indulged while he conducted a staggeringly brutal eight-year war of aggression against Iran; by subsequently attacking Kuwait, however, he demonstrated a sufficiently ravenous appetite for armed aggression that he became a clear threat to regional stability, whose removal had become (pace the US administration of the time--and several of its retired alumni today) a necessity. The later discovery of his huge unconventional weapons programs only reinforced the case for eliminating him--a case which was already compelling in 1990, and continues to persuade to this day (subject to the concerns I discussed previously, of course).