Handwaved answer to your first question is that the national
identity as a Big Thing likely emerged as a result of a 30-year-
war (more specifically, Peace of Westphalia), with nation-state
being an efficient unit for maintaining balance of power that
the Westphalian system demanded. To quote from Wiki: The edicts
agreed upon during the signing of the Peace of Westphalia were
instrumental in laying the foundations for what are even today
considered the basic tenets of the sovereign nation-state. Aside
from establishing fixed territorial boundaries for many of the
countries involved in the ordeal (as well as for the newer ones
created afterwards), the Peace of Westphalia changed the
relationship of subjects to their rulers. In earlier times,
people had tended to have overlapping political and religious
loyalties. Now, it was agreed that the citizenry of a respective
nation were subjected first and foremost to the laws and whims
of their own respective government rather than to those of
neighboring powers, be th