Max Fisher explains the origins of national identity in this excellent five minute clip for the New York Times. He notes that the idea of a national identity is a relatively new idea.
Just before the French Revolution, for example, France was not really a nation. Half the people could not even speak French. Ethnicity did not line up with borders either.
Over time, the idea that language race, and borders should equal a country developed. And then nations began to create myths to suggest that their nation always existed.
Check it out. This short clip might help students understand the importance of nationalism.