Pride in one's (blank) contributes to feelings of nationalism.

The best answer would be: Ethnicity

I believe the answer is: Ethnicity.

This sense of nationalism could make members of a certain civilization to unite in order to fight for a certain cause. (the cause could be either good or bad).

One example of the bad cause is when Hitler use the pride of Caucasian ethnicity to wiped out other races that he deemed as inferior.