For some years after his accession (936) Otto I was harassed by pretenders of his own family who allied themselves with one or more of the great Dukes. The Bavarians threatened to secede and form an independent nation; the Franconians rebelled when their right of waging private wars was called in question; the Lotharingians intrigued to make themselves an independent Middle Kingdom. All such malcontents found it easy to secure a brother or a son of the King as their nominal leader. Even when Otto had placed all the duchies in the hands of his own kinsmen or connections, his power was still precarious. For he claimed new rights which, though necessary to the maintenance of kingly power, did violence to feudal and provincial sentiment; while the Dukes whom he nominated usually took up the pretensions of their predecessors, and identified themselves with the interests of their subjects. It was more important than ever that the King should have the help of the clergy in educating public opinion. But in the most critical period (939-955) of the reign the German primate, Archbishop Frederic of Mainz, lent the weight of his influence and high personal reputation to the rebel cause. In another direction also Otto found the clergy the chief opponents of a cherished scheme. Organised missions were among the means on which he relied for civilising and extending his father's conquests in Slavonic territory. For this purpose he planned, with the approval of Rome, to make Magdeburg an archbishopric and the head of a Slavonic province. To this proposal the sees of Mainz and Halberstadt offered strenuous resistance, on the ground that it would curtail their jurisdictions (955). Twice, therefore, Otto had been sharply reminded that his authority over the German Church was insufficient for his purpose.