The general rules are that assassinating of leaders of a country are a no-go for any modern country. partially because of the extreme retaliation that would result, partially because you open the door to your own leadership being assassinated. Of course plenty of governments have been accused of secretly planning, or even successful pulling off, an assassinating before, but nothing publicly acknowledged.
I'm wondering if any large governments, especially more modern ones, have had an open and explicit policy saying that they usually wouldn't assassinate other world leaders unless that leader does some policy. I don't really care what the policy was, only that they had an explicit "you're fair game if you do this" sort of policy.
So for example in theory you might have some country warn that anyone who used nuclear weapons, or perhaps used them in a first strike situation, is now open to assassination attempts to discourage more dictatorial countries from having a single crazed leader starting a nuclear war.
Again I don't care what policy makes one fair game for assisination, only if there was an explicit warning that the action would make you fair game. I'd actually settle for a any semi explicit warnings, you know one of those we totally don't assassinate anyone, but we still highly advise you rethink your plans for your own good wink wink, nudge nudge.