The Crusades is the name given to a series of conflicts between Christian Europe and the Islamic Empire of the Near Middle East between the eleventh and fourteenth Centuries. The primary goal of the Crusades was to retake the city of Jerusalem and the Holy Land of Palestine from the "infidels." It had varying degrees of success. 

The First Crusade was able to successfully take Jerusalem and a few other isolated areas of the Holy Land, but the people who remained there were alone in a large area surrounded by hostile Arab enemies. They required constant resupplying from the trading ships of the Italian city-states, who subsequently grew immensely wealthy from the trading relationship.

The Holy Land and Jerusalem, captured in the First Crusade, were conquered by the Islamic forces of Saladin in 1187 CE. This led directly to the Third Crusade, waged from 1189 to 1192 CE, which did not result in the European forces retaking Jerusalem; however, it did lead Saladin to open the city of Jerusalem to protected Christian pilgrimage.

In the third century CE, the Roman Emperor Diocletian divided the Roman Empire into a Western Empire, centered around Rome, and an Eastern Empire, centered around Byzantium (later Constantinople, later still Istanbul). While the Western Roman Empire succumbed to barbarian invasion within two centuries, the Eastern Roman Empire continued until the fifteenth century when it was finally overcome by the Ottomans.

Harun al-Rashid is one of the most famous caliphs of the long lasting Abbasid Caliphate. He ruled from 786 C.E. until his death in 809 C.E. His reign is remembered as the height of the early Islamic golden era, marked by a flourishing of arts and science and characterized by continuous expansion of Islam. A caliph, just to be clear, is a particular type of ruler within an Islamic government said to be the political and religious successor to the Prophet Muhammad. 

Suleiman the Magnificent is one of the most famous rulers of the Ottoman Empire. He came to power in the early sixteenth century and is famous for his conquest of many Christian strongholds in Southern Europe, such as Belgrade, Budapest, and Rhodes.

The Abbasid Caliphate emerged in the eighth century C.E. as the inheritors of Muhammad's Islamic Empire. At the height of its power, the Abbasid Caliphate stretched from Central Asia through North Africa. The capital of the Abbasid Caliphate, for most of its existence, was Baghdad, in modern-day Iraq.

Constantinople was the capital of the Eastern Roman Empire, and then after the fall of the Western Roman Empire, the capital of Byzantium. The Byzantine Empire waned in influence from the 1100s to the 1400s, and by the time of Ottoman invasion in the fifteenth century, it was little more than a city-state. The Ottomans captured Constantinople in 1453 and renamed it Istanbul (the name of the city today).

Like so many other great empires in the period from 1200 to 1400, the Abbasid Caliphate was devastated by the attacks of the nomadic horsemen from the Mongolian Steppes—the Mongols. The Mongols sacked the Abbasid capital city, Baghdad, in 1258, bringing to an end the Golden Age of Islam.

All of these regions and kingdoms became Islamic during the time period from 500 C.E. to 1500 C.E. except for the Kingdom of Ethiopia, which remained a stronghold of Christianity and continues to be predominantly Christian to this day.