Late Medieval Islam
In 750 AD, it seemed as though the Islamic Empire would last a thousand years. The Abbasid caliphs ruled their giant empire, stretching from Morocco to Afghanistan, from their capital at Baghdad in modern Iraq. Baghdad was also a center of scholarship and trade. The Abbasids kept control until the early 900s - a run of more than two hundred years, which isn't bad for an empire. But then the empire split into two caliphates (KAL-if-fates). First the Fatimid dynasty took over all the Islamic land that had once been in the Roman Empire: Egypt, North Africa, Israel and Syria, and the Arabian Peninsula. The Abbasids still ruled the half that had once been the Sassanian Empire.
Then a second change began: Turkish and Mongol people from Central Asia, possibly pushed by some sort of climate change, began migrating south and west into both of the Islamic Empires. First, in 962 AD, the Ghaznavids broke off and made Afghanistan and Pakistan an independent country under their rule. Then just after 1000 AD, the Seljuks, who had come to the Islamic Empire as mercenary soldiers for the Abbasids, decided to rule for themselves instead. The Seljuks conquered the Ghaznavids, and by 1055 they also took over Baghdad, and in 1071 they conquered Turkey from the Byzantines at the Battle of Manzikert. The Seljuks spoke Persian, rather than Arabic.
The Islamic Empire's weakness invited other raiders in: in 1096 AD, European Crusaders conquered a good deal of Israel and Lebanon from the Fatimids and plundered it. By 1200, two more Turkish groups, the Ayyubids (under Saladin) and the Mamluks, reclaimed Israel from the Europeans. But more pieces broke away: the Almohads succeeded in forming an empire out of North Africa and Spain.
During the 1200s, the Almohad empire in turn broke apart into even smaller pieces. In northern Spain, the kingdoms of Aragon, Castile, and Portugal defeated the Almohads in 1212, and conquered most of Spain by 1248 AD. In North Africa, the Almohad empire also split into three smaller kingdoms: the Hafsids in the east, the al-Wadids in the center, and the Marinids in the west. During the 1300s and 1400s, the armies of Aragon and Castile gradually forced the Almohads and then the Nasrids out of Spain, finishing up in 1492 AD.
Back in the eastern part of the Islamic Empire, the Mongols invaded in 1260 AD - were they driven by the beginning of the Little Ice Age? - and conquered it from the Seljuks. The Mongols created another huge empire, but this one had different borders: it covered the old Sassanian Empire and Turkey, but also most of Central Asia, Pakistan, part of northern India, and China. The Mongols held their empire together for almost a hundred years, until the Black Death caused a collapse in the 1340s AD. Even after that, the Mongol Timur rebuilt part of the Mongol Empire, ruling from Turkey all the way across Russia to Afghanistan and northern India. But his empire collapsed when he died in 1405 AD.
In the 1400s, the Ottomans (successors to the Seljuks) began to build themselves a third big Islamic Empire. Like the Fatimids, the Ottomans focused on taking over the Roman half of the Islamic Empire. The Ottomans took Constantinople (modern Istanbul), Greece, and Eastern Europe from the Byzantines in 1453 AD. In 1464, they conquered the Marinids in Morocco. In 1517, they conquered Syria, Israel, and Egypt. In 1574, the Ottomans conquered the Hafsids in North Africa. So in these 800 years several big empires rose and fell, but in the end, most of the Islamic world was still united.