After Prussia was victorious in the Austro-Prussian War, Bismarck played a waiting game where the unification of Germany was concerned, as the joining of the southern states - initially resistant to Prussian rule, friendly with Austria, and bent on independence - would have to be overcome. What was needed was “a clear case of French aggression” toward either Prussia or the southern states. Not only would such a move by Emperor Napoleon III trigger the terms of the treaty between the German states, but it would keep the remaining world powers out of the conflict. 

The Franco-Prussian War started in August 1870, and a number of victories followed for the Prussians in battles in northeast France. By September, the strategic city of Metz was under siege, and forces fought a major battle at Sedan. Led by Field Marshal Helmuth von Moltke, the Prussians forced the French to surrender at Metz, and then at Sedan. Emperor Napoleon III, commanding his country’s forces at Sedan, was taken prisoner, humiliating France and its impetuous leader.