Nationalism encourage people with a shared identity to

separate themselves from those who were unlike them

Explanation:

As nationalism emerged in kingdoms and other regions of the world, where previously the notion of being a group with a common past, culture and history, and the shared language similarities did not exist before.

A conscience of sharing this heritage make them different to other groups that promoted other set of values.

Nationalism was driven often by romantic feelings, or revolutionary ideals, like the wave that schocked European continent with Napoleon Bonaparte.

The desire of people to have a shared identity often led to social movements that wanted to form their own nation-state, also leading to wars of Independence: (For example Serbia, and Bosnia in the Austro-Hungarian Empire). Poland sought always independence from the Russian and German oppresors (embracing the so called Romantic - Nationalism).