Not sure about this. Though clearly Japan wasn't treated as a western nation, even in its isolationist period, Japan was treated as an advanced nation. Europeans - and the USA - treated with Japan by sending diplomatic missions, trying to open it up to trade. Deciding on the spur of the moment to attack it would be a serious faux pas. It would be used as a sign of untrustworthiness by other European countries, it would destroy Prussia's foreign policy as Europe would instantly take steps away from them, and it would invite other European colonial powers to step in to help the Japanese, knowing that by helping them stay independent the Japanese would then owe their helpers a huge favour, meaning Most Favoured Nation status, huge trade concessions, thus huge profits, a lot of curried favour diplomatically, etc. Remember that the UK and (to a lesser extent) France were trading nations. Their prime interest was in making profits by trade. Also, Prussia had virtually no naval ability, so its ability to invade Japan would be nearly nonexistant. Japan really isn't a very likely target imo. Indochina is somewhat more likely, though, if they can find a way to transport troops.