Soon after disputes over the succession arose between the family of Guðröðr and that of an earlier king Harold. Ultimately the contest resolved itself into one between the sons of Guðröðr, especially one Horic (O.N. Hárekr) and a certain Harold. It lasted for several years, the sons of Guðröðr for the most part maintaining their hold on Denmark. At one time during the struggle Harold and his brother Ragnfröðr went to Vestfold in Norway, 'the extreme district of their realm, whose chiefs and peoples were refusing to be made subject to them, and gained their submission,' showing clearly that at this time Denmark and Southern Norway were under one rule and rendering probable the identification of Guðröðr with Guðröðr the Yngling who about this time was slain by a retainer in Stifla Sound on the south coast of Norway. This king ruled over Vestfold, half Vingulmörk and perhaps Agðir. Both parties were anxious to secure the support of the emperor Lewis and in the end Harold gained his help by accepting baptism at Mainz in 826. He promised to promote the cause of Christianity in Denmark, while Lewis in return granted him the district of Riustringen in Frisia as a place of retreat in case of necessity. The Danes thereby gained their first foothold within the empire. Sufficient has been said of the relation between Denmark and the empire on its land boundary: we must now say something of the attacks made by sea. The first were made in 799 on the coast of Aquitaine and they were probably due to raiders from Ireland who followed a well-known trade route from South Ireland to the ports of Southern France. In 800 Charlemagne inspected the coast from the Somme to the Seine and gave orders for the equipment of a fleet and the strengthening of the coastguard against Northmen pirates. When Guðröðr's fleet plundered the islands off the Frisian coast in 810, Charlemagne gave orders for his fleet to be strengthened once more, but the results were meagre in the extreme. The passage of the Channel was no longer safe, and year after year, from some time before 819, Vikings harried the island of Noirmoutier at the mouth of the Loire, commanding the port of Nantes and the extensive salt-trade of the district. The Island of Rhé opposite La Rochelle, was raided in similar fashion.