What final event caused the development of the political system known as feudalism?

The fall of the Roman Empire brought about feudalism. Before the fall, the poor people depended on the Emperor's soldiers to protect them. With the fall of the Roman Empire, the poor peasants turned to the lords of the land for protection. In return they allowed the lords to rule them and most of them became serfs to the lord. 

What war are you specifically talking about?

Italy france germany