At what point does the extent of
nationalism become harmful to the people
of the nation?

leads to conflict with others, Infringe on rights to others, creates xenophobia

the fear that some one will take them over, and the ideology of the nationalism speard and fostered new communal identities. it fostered a sense of patriotism and national pride. In many cases it also led to more cohesive society.

mesopotamian code of hammurabi

and i think its 1860 b.c. you mean lol