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.
With this popular enthusiasm Bismarck had no sympathy; and he had no interest for the cause of Schleswig-Holstein. He had originally considered the inhabitants merely as rebels against their lawful sovereign. He had learnt at Frankfort sufficient to make this indifferent to him, but he still regarded them as foreigners and looked on their claims merely from the point of view of Prussian interests. Both his sympathy and his reason led him in fact rather to take the Danish side. "The maintenance of Denmark is in our interest," he wrote in 1857, but Denmark could only continue to exist if it were ruled, more or less arbitrarily, with provincial Estates as it has been for the last hundred years; and in another letter: "We have no reason to desire that the Holsteiners should live very happily under their Duke, for if they do they will no longer be interested in Prussia, and under certain circumstances their interest may be very useful to us. It is important that, however just their cause may be, Prussia should act with great prudence." He recognised that if the complaints of the Duchies led again to a war between Germany and Denmark all the loss would fall on Prussia; the coast of Prussia was exposed to the attacks of the Danish fleet. If the war was successful, the result would be to strengthen the Diet and the Federal Constitution; and, as we know, that was the last thing which Bismarck desired; if it failed, the disgrace and the blame would fall upon Prussia.