Please go through the dawn article by Khuram Husain (25th of December).
http://epaper.dawn.com/DetailNews.ph...2_2014_008_006
Even though it does not provide a vivid reply to your question, it will substantiate your understanding of the underlying topic.

In my opinion, sectarianism has always existed in the history of Islamic countries because,

1: It is the most appropriate tool for effectively implementing the agendas of the political parties- the parties that base their ideology on the most strict and extreme interpretation of the Islamic laws. You will find a handful of religious parties that do not incorporate the use of dirty politics.
2: Muslim rulers had always engaged in power politics, therefore, the identity obtained via the realization of the concepts of a 'nation' or 'brotherhood' have always been overpowered by the dirty politics of regional domination.
3: Most of the Muslim majority countries are largely devoid of the essential factors that breed nationalism. For the ones that depict the opposite trend, various forms of governments have always been weak and fragile. Hence, the follow up of military based rules in the place of democratic government had resulted in ideological differences. Resultantly, in order to maintain their self-preservation, the concepts of sects emerged from the dogmas of parties whose prime purpose limited only to the rebellion by the general public.

You can search and quote some versus and hadiths that accentuate the importance of unity and combined duty.

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The one who is most deserving of thanks and praise from people is Allah