Final answer:
The ninth-century invasions and migrations caused a shift in the Byzantine-Arab relations, widespread destruction by the Mongols, and precipitated the downfall of economic prosperity in affected areas. In Europe, it gave rise to an era of disorder and violence, rooted in the feudal system's reliance on warfare and exacerbated by Viking, Magyar, and Arab invasions.
Step-by-step explanation:
The ninth-century invasions and migrations had profound and enduring consequences for European society. The relationship between Byzantines and Arabs shifted, as tribes like the Ghassanids lost the special privileges and allowances they previously enjoyed from the Byzantine Empire.
Theophanes, a Byzantine chronicler, detailed disruptions such as Arab tribes no longer receiving their allowances, marking the beginning of a change in regional dynamics.
Furthermore, the Mongol invasions led to widespread destruction of cities and cultural assets, including the burning of the Islamic Empire's collective knowledge stored in libraries. As Western European nations discovered new sea trade routes, regions impacted by the Mongols faced economic decline.
In Europe, invasions by the Vikings, Magyars, and Arab forces known as Saracens introduced prolonged periods of disorder and violence, upheaving the political landscape. The decentralization of power led to a feudal system based on violence, where knights and lords often initiated raids for livelihood, and civil wars could be incited by minor slights.