Why is nationalism considered bad in Islam?

Nationalism is favoring one’s nation based on the concept that it is superior to others. The feeling of pride is prohibited in Islam. Abdullah bin Mas'ud narrated that the Messenger of Allah (ﷺ) said, “None shall enter the Fire (of Hell) who has in his heart the weight of a mustard seed of Iman and none shall enter Paradise who has in his heart the weight of a mustard seed of pride.”[1]

Satan responded to the command of Allah to prostrate to Adam by saying,

“I am better than him.” [2]

He said this because he was created from fire and Adam from clay.

This pride, which is based on tribalism or racism where one simply regards oneself as superior because he belongs to it, is prohibited.

The Prophet (ﷺ) said, “Whoever fights for a cause that is not clear, advocating tribalism and getting angry for the sake of tribalism, then he has died a death of Jahiliyyah (pre-Islamic ignorance).”[3]

We have to get rid of tribalism for it is corruption and causes disunity between Muslims on false grounds. Tribalism, nationalism and racism are all prohibited in Islam.

“And verily this Brotherhood of yours is a single Brotherhood and I am your Lord and Cherisher: therefore fear Me (and no other).”[4]

References:

[1] Saheeh Muslim, 1:165

[2] Surah Al-Aaraf, 7:12

[3] Sunan An-Nasa'I, Hadith: 4115

[4] Surah Mu’minoon, 23:52