In the 11th century, populations north of the Alps began a more intensive settlement, targeting "new" lands, some of which areas had reverted to wilderness after the end of the Western Roman Empire. In what historian Charles Higounet called the "great clearances", Europeans cleared and cultivated some of the vast forests and marshes that lay across much of the continent. At the same time, settlers moved beyond the traditional boundaries of the Frankish Empire to new frontiers beyond the Elbe River, which tripled the size of Germany in the process. The Catholic Church, which reached the peak of its political power around then, called armies from across Europe to a series of Crusades against the Seljuk Turks. The crusaders occupied the Holy Land and founded the Crusader States in the Levant. Other wars led to the Northern Crusades. The Christian kingdoms took much of the Iberian Peninsula from Muslim control, and the Normans conquered southern Italy, all part of the major population increases and the resettlement patterns of the era.