Complex sentence: With the end of the Roman Empire and the following commercial crisis, Fezzan began to lose importance: the population was greatly reduced because of the desertification process of the Sahara during the early Middle Ages.
Split into simpler sentences: With the end of the Roman Empire and the following commercial crisis, Fezzan began to lose importance. The population was greatly reduced due to the desertification process of the Sahara during the early Middle Ages.