What was the initial issue that created a division for the Muslims?

Answer 1

It depends on how the question is understood.

Disagreements between Muslims

If the question is asking purely about disagreements between different groups of people where both groups are Muslims, as many issues split Muslims apart as any other group of people. There are racial issues in some places, gender issues in some place, political leadership issues in some place, theological issues in some place, labor issues in some places, etc. There are 1.6 billion Muslims which means that for any particular issue, you will have Muslims on both sides of the coin (just by statistics alone). Of course, there are some issues, like a man's right to marriage where debate is very minimal, but there are disagreements between Muslims over minute details of the ceremony or the age of consent, etc.

Prevention of Muslim Unity

If the question is what is preventing some sort of united Muslim government (i.e. Caliphate) or real intenational cooperation between all Islamic States, most Muslims chalk it up to power and greed of individual Muslim political leaders as well as "Western Imperialism". Of course, these rationales are fatuous and superficial. To see the real reasons, read on.

There are numerous things that prevent Muslim unity in the world.

Concerning Western Imperialism, Colonialism, and Warmongering: Many Muslims hold that the failure for Muslims to be united under one political banner has to do with western forces which divided and conquered them. There is certainly an extent to which that is true, but the Muslims had 25 separate Empires long before the Western regime of imperialism and colonialism. The first break in the unified Islamic State came only 120 years after its founding (The splitting off of Umayyad Spain and Idrissid Morocco). 50 years later saw the independence of the Aghlabids in Algeria, Tunisia, and Malta and the Buyids in Persia. 100 years after that saw the rise of the Fatimids in Western Libya, etc. By the mid-900s C.E. on (as Europe remained aloof from Islamic affairs except in Spain), Muslim-on-Muslim warfare and attempts for economic dominance persisted until the Ottomans conquered all major Islamic States in the Middle East. This was undone by separatist governors in the more distant provinces (like Algeria and Tunisia) and open rebellion in some of the closer provinces (like Hejaz and Syria). Some of these were supported by Western powers (like Hejaz). Others were crushed by them (such as the Syrians).

Concerning Historical Disunity:Muslims have gone to war with each other at numerous times and in numerous places. Nearly every border in the Arab World has played host to a military engagement of some type within the last 100 years. The Modern Arab World has never had anything even close to the European Union or the Schengen Border Agreement. (The Arab League is much more like the United Nations and has no superstate components.) Islam is an even weaker union that has no unified monetary policy, immigration policy, international policy (except perhaps Israel), and no unified interpretation of which version of Islam is the one worth supporting. Although Muslims, as individuals, may feel connected across borders, like those between Morocco and Algeria or those between Syria and Lebanon or those between Egypt, Sudan, and Libya, or those between Pakistan, India, and Bangladesh, or those between Malaysia and Indonesia, their governments do not.

Concerning National Cultural Differences: Each Muslim nation has a different ethnic, religious, and tribal makeup. Take for instance the neighboring countries of Tunisia and Libya. President Bourghiba of Tunisia proposed a union with Muammar Qaddafi of Libya, but his own people made it clear that they were not interested. The reason for this was that Tunisia was (and still is) the most progressive state in the Arab World, embracing such concepts as "national identity", "freedom of religion", near-complete "freedom of speech", and a Western-Style Government. Libya was (and still is) one of the most conservative Arab States outside of the Arabian Peninsula. Its people were very tribal with numerous militias sprinkled across the country and a system of bribes and counter-bribes used to keep the tribes from breaking out in revolution. (It seems that this federal bribing system eventually failed but the Libyan government has not managed to eclipse the militas.) With national organizational and cultural differences that were that huge, Pan-Arabism could not begin, let alone Pan-Islamism.

Concerning Regional Superiority: Each leader in the Muslim World believes that they are smarter, more capable, and more deserving of leadership than every other. However, uniting under one banner requires that there only be one government. This means that every other state must subordinate their power to one leader. Many Arabs joke about this bemusedly because of a conversation between Nasser of Egypt and Assad of Syria where Nasser said, "I am proud leader of the United Arab Republic based out of Cairo." and Assad said the next day, "I am proud leader of the United Arab Republic based out of Damascus." Both were referring to the same state, but each refused to really cede all control to the other. The United Arab Republic fell apart after three years. (And this was the longest period for such a united state.)

Concerning Oil Wealth: The Leaders of the Persian Gulf States such as Kuwait, Qatar, the United Arab Emirates, and Oman have little interest in giving up all of their oil wealth and power to accede to a vague hope and dream. This is especially true because it can be assumed that the leaders of such small states would definitely not become the leader of a united Islamic government.

Theological Differences

If the question is asking what theological disputes broke Islam apart into several different major sects (Sunni, Shiite, Ibadi, Ahmadi, Kharijite, Mu'tazilite - the last two now extinct, etc.),what happened to Islam was that various events, individuals, and ideas led to various kinds of divisions. These divisions are sometimes interpreted as breaks with orthodox teaching, and so various sects take root. The divisions can form around issues of scriptural interpretation, the application of various teachings, or the nature of authority in the religion and how that is going to be passed down. There are numerous splits in Islam, these are probably the most historically important.

The Sunni-Shiite Split

The main reason for the split here was: Disagreements over Succession after Mohammed's death.

The Muslim community was united while Mohammed was the leader of this community. Most Muslims hold, however, that he never specifically chose a successor to his leadership. There was a minority in the community that supported the candidacy of 'Ali, the Prophet's son-in-law, this political faction became known as the "Supporters of 'Ali" which in Arabic is Shiat 'Ali (where the modern term "Shiite" comes from). They derived their support from specific hadiths and events that they claimed showed that God had revealed to Mohammed that 'Ali would succeed him. The majority of Muslims held that these hadiths and events showed nothing more than that 'Ali was very pious, something they did not deny. Therefore they gave power to the man who was Mohammed's second-in-command and father-in-law Abu Bakr. This majority were called the People of the Customs [of the Prophet] which in Arabic is Ahl Sunna (from where the modern term "Sunni" comes from.)

The Sunni & Shiite-Kharijite/Ibadi Split

The main reason for the split here was: Impurity of Leadership.

The Kharijites were dominant in Egypt at the same time that the Shiites and Sunnis saw their first major confrontation: the Fitna Al-Kubra. At this point, the Kharijites fought along with the Shiites against the Sunnis and were able to press the Sunni leader, Mu'awiya, to the precipice of defeat. However, Mu'awiya asked to treat with 'Ali, the Fourth Rightly-Guided Caliph and leader of the Shiites. 'Ali's accepting of this invitation and the trickery that Mu'awiya played on him during the meeting deeply angered the Kharijites who believed that 'Ali was too easily deceived to be a true leader of God. They abandoned the Shiites and became a third party to the conflict. This made it possible for Mu'awiya to emerge victorious and conquer the Shiites. He then turned his attention to Kharijites and routed them. Kharijites disappeared completely a few centuries later, but they were succeeded by a less puritanical subgroup called the Ibadis who form the majority in Oman.

The Sunni-Mu'tazilite Split

The main reason for the split here was: Whether Islamic Law should be viewed through the lens of rational thought or purely Divine Edict.

The Mu'tazilites developed as a school in Baghdad that taught that a rational outlook on the Qur'anic traditions and the discussion of God, postulating a creation of the Qur'an, actual time-bound actions of God, and momentary attributes. In contrast, mainstream Sunni Islam at the time endorsed the idea that everything about God was co-eternal with Him (including any speech he made or books he revealed). While Mu'tazilites had power in Baghdad for roughly 200 years, they slowly waned and were "outvoted" by traditional Sunni theology.

The Sunni & Shiite-"Modern Prophets Islam" Split

The main reason for the split here was: Whether a Prophet could arise after the Death of Mohammed.

There have been a number of different Islamic Sects (like the Druze, the Baha'i, and the Ahmadiyya) who disavow the Muslim claim that Mohammed was the final prophet and the seal of the prophet. This betrays one of the fundamental claims of Islam. In order to avoid scrutiny and persecution about this, the Druze and Baha'i now self-advocate as different non-Islamic religions. The Ahmadiyya, however, still claim to be Muslim and are subject to violence due to their "apparent heresy".