Go cross the American border to Mexico and tell me it's not a physical manifestation of 2 separate nations
Discussion
That example is of nation-states. The Irish nation has more people outside of the island of Ireland than inside.
One could say the nation of Christians has no borders, the Bosnians are nationals without a state.
If aliens were watching us from space they would be unable to tell how many “nations” there are because it only exists in our minds. They could say that humans in this area look different than from that area but not why one human belongs to nation A and the other belongs to nation B. Do you know what I mean?