The Frankish empire was free from attack between the years 814 and 833. During the same time the English coast was also unvisited, and it is probable that the struggles for the succession in Denmark had for the time being reduced that kingdom to inactivity. About the year 830 the Danish king Hárekr seems to have established himself firmly on the throne, while on the other hand the emperor Lewis was troubled by the ambition of his sons Lewis, Pippin and Lothair. It is probably no chance coincidence that these events synchronised with the renewal of Viking attacks on Frisia. Throughout their history the Vikings showed themselves well informed of the changing political conditions of the countries which they visited and ready to make the utmost use of the opportunities which these might give for successful invasion. Frisia was the main point of attack during the next few years. Four times was the rich trading town of Duurstede ravaged; fleets sailed up the Veldt, the Maas, and the Scheldt; Antwerp was burned and the Island of Walcheren plundered, so that by the year 840 the greater part of Frisia south of the Vlie, was in Danish hands and so it remained till the end of the century. The Danish king Hárekr repeatedly denied all complicity in these raids and even promised to punish the raiders, but it is impossible to tell how far his denials were genuine. Equally difficult is it to say how far Harold in his Frisian home was responsible for these attacks. The annalists charge him with complicity, but Lewis seems to have thought it best to bind him by fresh gifts and (probably about 839) granted the district around Duurstede itself to him and his brother Roric (O.N. Hroerekr) on condition that they helped to ward off Viking attacks. All the efforts of the emperor to equip a fleet or to defend the coast were to no purpose, and there was even a suspicion that the Frisian populace were in sympathy with the Vikings. So great was the terror of attack that when in 839 a Byzantine mission, including some Rhôs or Swedes from Russia, visited the emperor at Ingelheim, the Swedes were for a time detained under suspicion, as spies. On the death of Lewis the Pious in 840 things went from bad to worse. The division of the empire in 843 gave the coast from the Eider to the Weser to Lewis, from the Weser to the Scheldt to Lothair, and the rest to Charles, removing all possibility of a united and organised defence, and soon these princes entered on the fatal policy of calling in the Vikings to assist them in their quarrels. Thus Lothair in 841 endeavoured to bind Harold to his cause by a grant of the Island of Walcheren and Harold is found in the following year with Lothair's army on the Moselle.