The gradual decline of the Roman Empire ushered in an era of European history called the Middle Ages. During this time period, the Frankish kingdom, under the leadership of Charlemagne, emerged as the strongest kingdom in Europe. Charlemagne expanded the borders of his kingdom, spread Christianity, and encouraged learning. The prospect of recreating the Roman Empire seemed possible until Charlemagne's death, when a series of barbarian invaders weakened Carolingian authority. The resulting chaos contributed to the rise of feudalism, which served as a system of mutual obligations to protect large groups of people from the invasions of foreigners. While invaders altered the culture of Europe, the Roman Catholic Church remained an unchanged and powerful institution.