Dying for a country is the existential foundation of the nation-state.

Dying and killing defines political existence. If there was "nothing to kill or die for" (John Lennon), then you might have the capacity to "imagine there was no country." 

What would historians write about?

"History books" require dying and killing for nations.

One can die for the United States or for China or for Korea, although Americans aren't hip on dying anymore.

The Germans, especially, are no longer hot to die and kill. They did it for Hitler, and it wasn't as much fun as they thought it would be.