What often led to alliances between European feudal lords?

Question

Quizplus · Accepted Answer

A mutual need for defense against the Vikings often led to alliances between European feudal lords.
Key Takeaway: The earliest recorded Viking raids occurred in the 790s and lasted until the Norman Conquest of England in 1066. This is generally known as the Viking Age of Scandinavian history. The Vikings went on to sail most of the North Atlantic, reaching south to North Africa and east to Russia, Constantinople, and the Middle East, looting, plundering, trading, and colonizing as they saw fit.
At the time, Europe suffered from internal divisions, and was relatively easy prey given the proximity of many towns to the sea or navigable rivers. Lack of organized naval opposition throughout Western Europe allowed Viking ships to travel freely, raiding or trading as opportunity permitted. To protect themselves against the Vikings, many European feudal lords often used to band together to defend their property.

What Often Led to Alliances Between European Feudal Lords