The international situation was favourable to an aggressive program of unification in the German Confederation. Since its defeat in the Crimean War (1853–56), Russia had ceased to play a decisive role in the affairs of the Continent. Britain remained preoccupied with the problems of domestic reform. And Napoleon III was not unwilling to see a civil war east of the Rhine that he might eventually use to enlarge the boundaries of France. Bismarck could thus prepare for a struggle against Austria without the imminent danger of foreign intervention that had faced Frederick William IV. His first great opportunity came in connection with the duchies of Schleswig and Holstein, which were ruled by the king of Denmark but which were politically and ethnically tied to Germany. When the government in Copenhagen sought to make Schleswig an integral part of the Danish state in 1863, nationalist sentiment in Germany was outraged. William I proposed to Francis Joseph that the two leading powers of the German Confederation should occupy the duchies in order to prevent the violation of an international agreement that had guaranteed their autonomy. Afraid to let the Prussians act on their own, the emperor agreed, and in 1864 the brief German-Danish War demonstrated the strength of the reorganized Prussian army. Danish hopes for foreign assistance proved illusory, and by the Peace of Vienna (October 30) the duchies became the joint possession of Prussia and Austria.

The easy victory of the allies, however, was only the prelude to a bitter conflict between them. Vienna would have liked to see Schleswig-Holstein become an independent secondary state in the German Confederation, committed to a policy of particularism. Berlin, on the other hand, hoped for the outright annexation of the duchies or at least the indirect control of their government. Even more important than the disposition of the spoils of war, however, was the mounting rivalry between the two Great Powers for hegemony in Germany. In 1865 their differences were papered over by the Convention of Gastein, which placed Schleswig under Prussian and Holstein under Austrian administration but which also reaffirmed the joint sovereignty of the two governments over the duchies. Still, this was only a temporary solution, and before long the danger of civil war in the German Confederation began to grow once again.

In the course of the spring of 1866 both sides stepped up their preparations for a military solution to the Austro-Prussian rivalry. Bismarck concluded an alliance with Italy by which the Italians were to receive Venetia as a reward for participating in a war against the Habsburg empire. He also sought to gain the support of public opinion in the German Confederation by introducing a motion in the federal Diet for the convocation of a national parliament elected by equal manhood suffrage. The Austrians in the meantime secured a promise of French neutrality in the event of hostilities and tried to win the adherence of the secondary states in the impending struggle. The last desperate attempts to preserve peace collapsed in June. Vienna announced that it would submit the question of the duchies to the federal Diet. Berlin, condemning this step as a violation of the Convention of Gastein, ordered its troops in Schleswig to expel the Austrians from Holstein. Francis Joseph in reply called on the other states of the confederation to mobilize their armies against the Prussian threat to domestic tranquillity, and Germany trembled on the verge of civil war. The only question now was the position of the secondary states. Most of them lined up behind Austria, which they regarded as the defender of their independence against the ambitions of Berlin. Bismarck’s attempt to enlist the aid of the national movement by advocating reform of the confederal system thus failed. It alarmed the particularists without propitiating the centralists. Public opinion remained frightened and confused, distrusting one side and fearing the other. The future of the nation was decided not by popular insurrections or parliamentary deliberations but by the force of arms.

The Seven Weeks’ War between Prussia and Austria (June–August 1866) produced a diplomatic revolution in Europe, destroying the balance of power that had been established 50 years before by the Congress of Vienna. Yet this momentous alteration in the international equilibrium was accomplished so swiftly that foreign diplomats had barely begun to grasp its implications before the struggle for hegemony in Germany ended. The Prussian armies had a brilliant strategist in Helmuth von Moltke and a deadly weapon in the breech-loading needle gun. The Austrian high command, on the other hand, became irresolute and demoralized before a decisive encounter had even taken place. The Prussians succeeded in dividing and defeating the forces of the secondary states, and on July 3 they routed the Habsburg troops as well at the Battle of Königgrätz (Sadowa). The war was thus decided within a few weeks after its outbreak. Bismarck, refusing to be dazzled by the brilliance of the victory, urged the swift conclusion of an honourable peace. Not only did he feel that the preservation of a strong Austria was essential for the maintenance of stability on the Continent, but he also feared that a prolongation of hostilities would enable Napoleon III to intervene in the affairs of Germany. By the preliminary Peace of Nikolsburg (July 26) and the definitive Treaty of Prague (August 23), Francis Joseph was permitted to retain all of his possessions except Venetia, which had been promised to the Italians. There was to be no occupation and only a modest indemnity. The emperor had to acquiesce, however, in the Prussian annexation of Hanover, Nassau, Hesse-Kassel, Schleswig-Holstein, and Frankfurt am Main, in the dissolution of the German Confederation, and in the formation under Prussian leadership of a new federal union north of the Main River. The contest between Berlin and Vienna that had determined the history of the German states for more than a century was now over.