The shift from centralized governance to smaller communities contributed to the large-scale pillaging that took place in the Middle Ages by groups like the Vikings.
In response, much of Europe began building fortifications in the form of castles. These were meant to both protect the lives of the wealthy, influential and their servants in the area in case of a raid and to make it difficult for them to be chased out of their manor¹.
The nobles of each feudal community were required to provide their own infantry and weaponry if there was a battle¹. One of the only ways to achieve any upwards social mobility in Middle Aged Europe was for knights to consistently respond to and be successful in calls to battle.
The Medieval Knight that most people associate with the Middle Ages with was a mounted soldier that typically had some association with nobility. This wasn’t always the case and depended on the region. The price tag on the horse, armor and weapons eventually led to the evolution of the knight into its own social class that would be the forces behind the crusades².
The crusades were a series of military and pilgrimage expeditions that took place from the 11th century to the 16th centuries from parts of Europe to the Eastern coast of the Mediterranean². These began primarily for Christians to take control over the Holy Land. This land had been in the hands of Muslims 638 CE¹ and it was towards them that the crusades were primarily directed.
The main crusades spanned from the 11th to the 13th centuries. During Europe’s Dark Ages, Islam had been expanding exponentially elsewhere. Medieval popes created the crusader as a Christian knight. The new social order of fighting monks and nobles was born in the Knights Templar¹.
References:
1. Crusades and Crusaders. Retrieved from http://www.medievalwarfare.info/crusades.htm
2. The Crusades - Pilgrimage or Holy War?: Crash Course World History #15. Retrieved from https://www.youtube.com/watch?v=X0zudTQelzI