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 period between the first-known Viking raids in the 790s and the Norman Conquest of England (1066) is usually referred to as the Viking Age. During this time, Vikings sailed (and largely controlled) much of the North Atlantic, moving as far south as North Africa and as far east as Russia and the Middle East. The Viking raiders were a relatively brutal group and often colonized, plundered, and looted any colonies they encountered.
At this time, Europe was very divided, internally, making it easy prey for outside raiders. There was no organized army or navy. Due to a mutual need for defense against the Vikings, European feudal lords often banded together to protect their land and peoples.

What Often Led to Alliances Between European Feudal Lords