The next impulse came from the edge of Europe. Although Ireland had never been part of the Roman Empire, Christianity had come there and developed, largely independently, into Celtic Christianity. The Irish monks had developed a concept of "peregrinatio". This essentially meant that a monk would leave the monastery and his Christian country to proselytize among the heathens. From 590 onwards, Irish missionaries were active in Gaul, Scotland, Wales and England. During the Saxon Wars, Charlemagne, King of the Franks, Christianized the Saxons by way of warfare and law upon conquest.