The Ems Dispatch (sometimes called the Ems Telegram) is the document that instigated the Franco-Prussian War. Bad Ems is east of Koblenz on the Lahn river. At the time Ems was in Prussia.
The French concern was that the German prince Leopold of Hohenzollern-Sigmaringen had been offered the Spanish throne. The French government was concerned over a possible Prusso-Spanish alliance between fellow Hohenzollerns. The throne had which had been vacant since September 1868, Leopold had withdrawn his acceptance in July after French protests. But the French demanded further commitments.
On July 13, 1870 King William of Prussia was interviewed at the Kursaal in Ems by Count Vincent Benedetti, the French ambassador in Prussia since 1864. Benedetti had been instructed by Antoine Agénor Alfred, the Duc de Gramont, to present the French demand that the king should guarantee that he would never approve the candidacy of a Hohenzollern to the Spanish throne. The meeting was informal and took place on the promenade of the Kursaal, the King refused to agree to the French demand "somewhat severely" but politely and the meeting ended.
From the meeting the King wrote an account and it was passed on to Otto von Bismarck. The King described Benedetti as "very importunate" but then, either the King gave permission to Bismarck to re-edit and release an account of the events to provoke the French or Bismarck took it on himself to do that. Certainly the edit of the telegram released on July 14 to the media and foreign embassies gave the impression that Benedetti was rather more demanding and the King exceedingly abrupt.
France declared war on July 19, 1870.
Benedetti published an account of the meeting in Ma Mission en Prusse (1871).