The question of constitution was complicated by one of succession.  The male line of the Royal House which ruled in Denmark was dying out; according to a law introduced in 1660, descendants of the female branch might succeed in the Kingdom.  This law had probably never been legally enacted for the Duchies; in Schleswig and Holstein the old Salic law prevailed.  In the ordinary course of things, on the death of Frederick VII., who had succeeded in 1847, the long connection between Holstein and Denmark would cease.  Would, however, Schleswig go with Holstein or with Denmark?  Every Schleswig-Holsteiner and every German declared that the two Duchies must remain for ever “unvertheilt”; the majority of the Danes determined, whatever the law might be, that they would keep Schleswig, which had once been Danish.  The King took a different line; he wished to maintain all the possessions in his House, and that the same man should succeed both in the Kingdom and the Duchies.  There was no authority qualified to decide the legal question; and therefore the question of right was sure to become one of power.  At first, strange as it may seem, the power was on the side of the Danes.  Germany was weak and disunited, the Prussian troops who had been sent to help the rebellion were withdrawn, and the surrender of Olmuetz was fatal to the inhabitants of the Duchies.  The whole question was brought before a European Congress which met at London.  The integrity of the Danish Monarchy was declared to be a European interest; and the Congress of the Powers presumed to determine who should succeed to the ducal and royal power.  They chose Christian of Glucksburg, and all the Powers pledged themselves to recognise him as ruler over all the dominions of the King of Denmark.

Prussia and Austria were among the Powers who signed the Treaty of London, but the Diet of Frankfort was not bound by it.  At the same time, Denmark had entered into certain engagements pledging itself to preserve the separation between Schleswig-Holstein and Denmark, and also not to oppress the German people in Schleswig.  The Danes did not keep their engagement; despising the Germans, they renewed the old policy, attempted to drive back the German language, and introduced new laws which were inconsistent with the local privileges of Holstein and Schleswig.  The Holstein Estates appealed for protection to the Diet.  The Germans protested, but the Danes were obstinate.  As years went on, the excitement of the Germans grew; they believed, and justly believed, that it was a matter of honour to defend the rights of the Duchies.  Schleswig-Holstein was the symbol of German weakness and disgrace, and in defence of them the national enthusiasm was again roused.