Nationalism in Islam
Nationalism means identification with one's own nation and support for its interests, especially to the exclusion or detriment of the interests of other nations.
Nationalism is contrary to Islam because this is calling of splitting ummah, it is similar to tribalism practiced during Day of Ignorance, undoubtedly, nationalism and tribalism both are sources of hatred, arrogance, and pride, and even it can affect marriage badly nationalists such as forbidding their children from marrying those not belonging to their homeland or of different race color.
Prophet peace be upon him warned us about fighting for sake of the tribe.
It was narrated that Jundab bin 'Abdullah said:
"The Messenger of Allah (ﷺ) said: 'Whoever fights for a cause that is not clear, advocating tribalism, getting angry for the sake of tribalism, then he has died a death of Jahiliyyah.'"
Sahih (Darussalam)
Sunan an-Nasa'i 4115
In-book: Book 37, Hadith 150
English translation: Vol. 5, Book 37, Hadith 4120
It was narrated from Abu Hurairah that the Messenger of Allah (ﷺ) said:
"Whoever fights under a banner of folly, supporting tribalism, or getting angry for the sake of tribalism, he dies in a state of ignorance."
Sahih (Darussalam)
Sunan Ibn Majah 3948
In-book : Book 36, Hadith 23
English translation : Vol. 5, Book 36, Hadith 3948
Sunan Ibn Majah
Indeed it does not matter what our race colors are, be it black, white, or brown, we are all children of Adam, who was created from dust. Our Lord is One and Only worthy of being worshipped alone without any partner.
Allah tells us
Al-Hujurat 49:13
يَٰٓأَيُّهَا ٱلنَّاسُ إِنَّا خَلَقْنَٰكُم مِّن ذَكَرٍ وَأُنثَىٰ وَجَعَلْنَٰكُمْ شُعُوبًا وَقَبَآئِلَ لِتَعَارَفُوٓا۟ۚ إِنَّ أَكْرَمَكُمْ عِندَ ٱللَّهِ أَتْقَىٰكُمْۚ إِنَّ ٱللَّهَ عَلِيمٌ خَبِيرٌ
O mankind, indeed We have created you from male and female and made you peoples and tribes that you may know one another. Indeed, the noblest of you in the sight of Allah is the most righteous of you. Indeed, Allah is Knowing and Acquainted.
And we should be aware that Allah does not look at our appearance or wealth but He looks at our heart.
May Allah protect us from the ugly nationalism and tribalism