Every counrty that has a modern military has some form of 'assassin' whether they are defined as an arm of the state intelligence gathering agency or a specialized unit of that nations military. As Japan had limitations placed upon it's military by the Allied powers in WWII I doubt that they have such a thing now. (unless the US has turned a blind eye twoards it's formation) I have read that some yakuza gangs have specially trained members that specialize in killing rivial mob bosses but most information regarding the yakuza is questionable at best. From a military history perspective I would say you posed an interesting question Kenshin.