Several things drove this expansion. The Vikings were drawn by the growth of wealthy towns and monasteries overseas, and weak kingdoms. They may also have been pushed to leave their homeland by overpopulation, lack of good farmland, and political strife arising from the unification of Norway. The aggressive expansion of the Carolingian Empire and forced conversion of the neighboring Saxons to Christianity may also have been a factor. Sailing innovations had allowed the Vikings to sail further and longer to begin with.