I'm not convinced it hasn't worked. There were rumors the US diplomats said to Putin that if he used nukes in Ukraine then probably the US would a) take out his

I'm not convinced it hasn't worked. There were rumors the US diplomats said to Putin that if he used nukes in Ukraine then probably the US would a) take out his military in Ukraine and b) target him to be killed personally. The diplomatic bit is saying that privately rather than actually getting the nuking and killing going.