structural:summarization
Hohenzollern candidacy and the Ems telegram

In his memoirs, written long after the war, Bismarck wrote, "I always considered that a war with France would naturally follow a war against Austria... I was convinced that the gulf which was created over time between the north and the south of Germany could not be better overcome than by a national war against the neighbouring people who were aggressive against us. I did not doubt that it was necessary to make a French-German war before the general reorganization of Germany could be realized." As the summer of 1870 approached, pressure mounted on Bismarck to have a war with France as quickly as possible. In Bavaria, the largest of the southern German states, unification with (mostly Protestant) Prussia was being opposed by the Patriotic Party, which favoured a confederacy of (Catholic) Bavaria with (Catholic) Austria. German Protestant public opinion was on the side of unification with Prussia.

In France, patriotic sentiment was also growing. On 8 May 1870, French voters had overwhelmingly supported Napoleon III's program in a national plebiscite, with 7,358,000 votes yes against 1,582,000 votes no, an increase of support of two million votes since the legislative elections in 1869. The Emperor was less popular in Paris and the big cities, but highly popular in the French countryside. Napoleon had named a new foreign minister, Antoine Agenor, the Duke de Gramont, who was hostile to Bismarck. The Emperor was weak and ill, but the more extreme Bonapartists were prepared to show their strength against the republicans and monarchists in the parliament.

The news of Leopold, Prince of Hohenzollern 's candidacy for the Spanish crown, published 2 July 1870, aroused fury in the French parliament and press. The government was attacked by both the republicans and monarchist opposition, and by the ultra-Bonapartists, for its weakness against Prussia. On 6 July, Napoleon III held a meeting of his ministers at the château of Saint-Cloud and told them that Prussia must withdraw the Hohenzollern candidacy or there would be a war. He asked Marshal Leboeuf, the chief of staff of the French army, if the army was prepared for a war against Prussia. Leboeuf responded that the French soldiers had a rifle superior to the Prussian rifle, that the French artillery was commanded by an elite corps of officers, and that the army "would not lack a button on its puttees ". He assured the Emperor that the French army could have four hundred thousand men on the Rhine in less than fifteen days.

The French Ambassador to Prussia, Count Vincent Benedetti, was sent to the German spa resort of Bad Ems, where the Prussian king was staying. Benedetti met with the king on 13 July in the park of the château. The king told him courteously that he agreed fully with the withdrawal of the Hohenzollern candidacy, but that he could not make promises on behalf of the government for the future. He considered that the matter was closed. As he was instructed by Gramont, Benedetti asked for another meeting with the king to repeat the request, but the king politely, yet firmly, refused. Benedetti returned to Paris and the affair seemed finished. However, Bismarck edited the official dispatch of the meeting to make it appear that both sides had been hostile: "His majesty the King," the dispatch read, "refused to meet again with the French ambassador, and let him know, through an aide-de-camp of service, that His Majesty had nothing more to say to the Ambassador." This version was communicated to governments, and the next day was in the French press.

The Ems telegram had exactly the effect that Bismarck had intended. Once again, public opinion in France was inflamed. "This text produced the effect of a red flag to the Gallic bull," Bismarck later wrote. Gramont, the French foreign minister, declared that he felt "he had just received a slap." The leader of the conservatives in parliament, Thiers, spoke for moderation, arguing that France had won the diplomatic battle and there was no reason for war, but he was drowned out by cries that he was a traitor and a Prussian. Napoleon's new prime minister, Émile Ollivier, declared that France had done all that it could humanly and honourably do to prevent the war, and that he accepted the responsibility "with a light heart". A crowd of 15,000–20,000 persons, carrying flags and patriotic banners, marched through the streets of Paris, demanding war. On 19 July 1870, a declaration of war was sent to the Prussian government.