Ems Dispatch

The Ems Dispatch (French: Dépêche d'Ems, German: Emser Depesche), sometimes called the Ems Telegram, incited France to declare the Franco-Prussian War in July 1870. The actual dispatch was an internal message from the Prussian King's vacationing site to Otto von Bismarck in Berlin, reporting demands made by the French ambassador; it was Bismarck's released statement to the press that became known as Ems Dispatch. The name referred to Bad Ems, a resort spa east of Koblenz on the Lahn river, then situated in Hesse-Nassau, a new possession of Prussia.

The 'Seven Weeks' Austro-Prussian War of 16 June - 23 August 1866, which involved south and north German states on both sides as well as an emerging Italy, increased Prussian power. Austria was defeated in the key Battle of Königgrätz (or Sadowa) on 3 July. The preliminary peace of Nikolsburg, 26 July, was followed by the Peace of Prague, 23 August. The result was that Bismarck was able to expel Austria from the German Confederation, to set up and dominate the North German Confederation—north of the Main—and secure Prussian territorial gains. Hanover, Schleswig-Holstein, Nassau and Hesse-Cassel, and Frankfurt were incorporated into the Prussian kingdom. Prussia's territory expanded by nearly a quarter and its population by more than four million to roughly twenty-four million, though still less than France's thirty-eight million. France did not take part in the war which, though brief, altered the European balance of power. France did not gain territories or prestige. French demands for a "Revanche pour Sadova" (Revenge for Sadowa) took root.

In early 1870, the German Prince Leopold, of the Roman Catholic cadet branch Hohenzollern-Sigmaringen, had been offered the vacant Spanish throne. The government of French Emperor Napoleon III voiced concern over a possible Spanish alliance with the Protestant House of Hohenzollern that ruled the Kingdom of Prussia, protested against it, and hinted about a war. Following the protests by France, Leopold had withdrawn his acceptance in July 1870. This was already considered a diplomatic defeat for Prussia. The French were not yet satisfied with this and demanded further commitments, especially a guarantee by the Prussian king that no member of any branch of his Hohenzollern family would ever be a candidate for the Spanish throne.

...
Wikipedia