Nationalism ideology is based in that the individual loyalty to a nation should surpass any other interest. In politics implies identification of a nation with the people. Nationalism can be dangerous as proved by the atomization of the Austro-Hungarian Empire into new small states which went on having serious internal conflicts.
It encourages arguments (like us vs. you). It makes others feel superior and selfish. It causes harm to countries and causes violence. A nationalist leader might lie to maintain superiority. Here's an example that might help: America is the greatest country on Earth. Hope I helped!
