Rasul Allah (salalaihi wa sallam) said: “He is not one of us who calls for asabiyyah, (tribalism/nationalism) or who fights for asabiyyah, or who dies for asabiyyah.” [Abu Dawud]
On one occasion during the time of the Prophet (sal Allahu alaihi wa sallam), a party of Jews conspired to bring about disunity in the ranks of the Muslims after seeing the Aus and Khazraj harmonious with each other within Islam. A youth from among them was sent to incite remembrance of the battle of Bu'ath, where the Aus had been victorious over the Khazraj. He recited poetry to stir their feelings, remind them of their past differences, and bring about division between them. As a result there was a call to arms. When the news reached the Messenger of Allah (sal Allahu alaihi wa sallam) he said, "O Muslims, remember Allah, remember Allah. Will you act as pagans while I am present among you, after Allah has guided you to Islam, and honoured you thereby and made a clean break with paganism; delivered you thereby from disbelief; and made you friends thereby?" When they heard this they wept and embraced each other. [The Roots of Nationalism In The Muslim World]
Islam is a religion that abhors racism, ethnicism, tribalism, and its newer version, nationalism. Differences of language, colour, cuisine and accent were meant to add flavour to life, not to be used to ridicule or fight each other. The past couple of centuries have seen wars between on a scale unprecedented before. The rise of nationalism has resulted in bitter conflict between groups that had lived for centuries in harmony. It has set the world on fire and resulted in pandaemonium.
While Allah (subhana wa ta'ala) has prohibited nationalism, Muslims are killing one another and aiding non-Muslims to kill their brothers and sisters in Islam, in the name of saving their own "country". This is despite the fact that such "countries" have no legitimacy in Allah's eyes for the reasons that 1) in none of these is Allah's law held wholly superior to man made law, and 2) there is only one country to which all Muslims belong, i.e. Dar-ul-Islam. Muslims, for thirteen centuries were one bloc, under one leadership, in the name of Allah (subhana wa ta'ala). When you cut an elephant into 100 pieces you don't get one hundred elephants, instead you get a bloody mess. Incited against each other using nationalism, this is what the Muslim Ummah has become.