If you’re talking about early Muslim empires, then the Rashidun Caliphate (the first Islamic caliphate) is pretty much essential to discuss, it had an enormous role in the development and spread of Islam. I think it would especially be important to talk about in your history class if you wish to connect it to current affairs, as the division between Sunni and Shia Muslims had its origins in the late Rashidun Caliphate, and this division is still felt today (especially between countries such as Saudi Arabia, Iraq, and Iran).

It is important to note there was no single title for the ruler of an Islamic kingdom or empire, there were different potential titles with different meanings. In the early 12th century, for example, there were multiple Islamic kingdoms, there was the Seljuk Empire and the Sultanate of Rum, which were both ruled by sultans, and the Abbasid Caliphate, which was ruled by a caliph, just to name a few. Though sultans and caliphs both acted as rulers and military leaders in their respective kingdoms, sultans are secular leaders, while caliphs acted as both secular and religious leaders. The caliphs saw themselves as successors to Muhammad. Though not a perfect example, I would sort of compare the difference between a sultan and a caliph to the Holy Roman Emperor and the Pope (the latter having much secular and religious power in the Middle Ages, the former being a secular leader over much of Western Europe) There were also several leaders who used multiple titles, such as later Ottoman rulers, who used the titles sultan and caliph until 1922. Claiming to be the successor of a prophet would, unsurprisingly, lead to conflict, which I will address later in my answer.

The tradition of the caliphate, as I have stated, goes back to the time of Muhammad in the early 7th century. Alongside being a religious leader (Muslims believe Muhammad to be the final prophet of God), Muhammad was also a military leader. Traditional biographies of Muhammad state the early Muslims were persecuted in Mecca for their religious beliefs, the Muslims, with Muhammad, migrated from the city of Mecca to Medina in AD 622. Muhamad was able to establish himself in Medina, and the rapid spread of Islam (which you have mentioned) began while Muhammad was still alive. Much of Arabia had converted to Islam during the time of Muhammad, eventually including even Mecca, who fought several battles against the early Muslims (some of these battles, such as the Battle of Badr in 624, are briefly mentioned in the Qur’an). Muhammad never appointed a successor, so there was much confusion over who should replace him militarily following his death in AD 632. Eventually, the Muslims did choose a successor, Abu Bakr, who was Muhammad’s father-in-law; Abu Bakr became the first caliph of what would become the Rashidun Caliphate. The caliphate lasted until 661 and had four caliphs: Abu Bakr, Umar, Uthman (who is notable for codifying the Qur’an into a single volume), and Ali, all four Rashidun caliphs personally knew Muhammad. The division between Sunni and Shia Muslims arose during the time of Ali; it is important to note that “Rashidun (or Rightly Guided)” is the term applied to the caliphate by Sunni Muslims, Shia Muslims reject this term. The disagreement between Sunnis and Shia arises from who can and cannot be called a caliph. Sunni Muslims believe the caliph does not necessarily have to be hereditary so long as they are a competent leader for the Muslims, Abu Bakr, Umar, and Uthman, therefore, are accepted as legitimate caliphs by Sunnis in spite of the fact they were not related to Muhammad. Shia Muslims, on the other hand, believe the caliphate should be hereditary, and that caliphs should be descended from Muhammad in some way. Muhammad left no surviving male children, but he did have living relatives, notably Ali, who was Muhammad’s cousin. Shia Muslims regard Ali, therefore, as the legitimate first caliph, and reject the caliphate of Abu Bakr, Umar, and Uthman. Ali was assassinated in 661 during a civil war between him and Muawiyah I, the latter took over the title of caliph, beginning the Umayyad Caliphate. Again, the Umayyad Caliphate is typically rejected by Shia Muslims for the same reason the Rashidun Caliphate is rejected, with Shias instead following the traditions of the imams (Ali and his direct descendants). The number of imams differs depending on different groups of Shia Muslims, with some believing there were nine imams, and others believing there were twelve (Shias who believe there were twelve imams are referred to as “Twelvers”). You could perhaps use this as an opportunity to talk about modern tensions between Sunni and Shia Muslims (to the extent you would be able to for a middle school classroom) such as accusations of one group or the other not being real Muslims. It would provide an excellent example to the students of how Islam is not a monolith, and how there are differing views and conflicts which are visible to any religious or social group.

You could probably also discuss how Islam was able to spread so rapidly in such a short amount of time. Much of this has to do with the extensive war fought between the Byzantine and Sassanid Empires from 602-628, just before the Rashidun Caliphate emerged. The caliphate was able to take advantage of the incredibly weakened status of both empires, and conquered much of the Byzantine and Sassanid territory, the latter empire, in fact, was wiped out entirely in 651 (my knowledge for caliphates following Ali’s death is very limited)

I am very glad to hear you are teaching your students not just European history, but also Middle Eastern history, I think learning about a variety of nations throughout history is very important! I also hope you have found my answer to be helpful, whether it is or is not able to influence what you will be teaching your students!

Sources

Abdel-Haleem, Muhammad, translator. The Qur'an. Oxford University Press, 2015.

Arnold, Thomas Walker. The Caliphate. Routledge, 2018.

Grant, Reg et al. The History Book. Dorling Kindersley, 2016.

Mark, Joshua J. “Yazdegerd III.” Ancient History Encyclopedia, Ancient History Encyclopedia, 18 May 2020, www.ancient.eu/Yazdegerd_III/.