I don't consider different eras as different nations though. I think that's splitting too many hairs. I see a nation as a country that is generally united and governed by a leading entity.
Going back to the Japan example, I would consider them a nation when all the clans were united under one rule. Same with UK, India, Thailand etc.
Well, I'm in Australia so I would try and communicate with the indigenous population and teach them how to defend themselves against the upcoming colonist invasion and take advantage of what they have to offer, starting with forcing a signed treaty.