Spain's revolution and the establishment of a Provisional Government drew the attention of many European nations.
After the Spanish Parliament announced that it would not abolish the monarchy and would seek a new king, many countries watched the spectacle with amusement while wondering which lucky man would become the new King of Spain.
But before long, the French government could no longer laugh.
According to news from Spain, among the many candidates for King of Spain, Prince Leopold of House of Hohenzollern ranked near the top and was highly likely to become the next King of Spain.
(There were several Prince Leopolds in Germany—that was, several Leopolds. The candidate for this throne was Prince Leopold of Hohenzollern, not the one from Bavaria. Prince Leopold of Hohenzollern was the father of Romania's King Ferdinand I.)
This was deadly to the french people. Prussia alone had already posed an enormous threat to France. Especially after defeating Austria, Prussia's threat to France was unprecedented.
If someone from House of Hohenzollern gained Spain's crown, France would face being attacked from both sides by Prussia and Spain.
Although Spain was not considered a great power and could barely count as a second-rate European nation, it lay south of France and possessed the Pyrenees Mountains as a natural defensive line.
Prussia only needed to station fewer than a hundred thousand troops in Spain to cause the french people immense trouble and threaten many cities in southern France.
On June 21, 1869, the enraged French government voiced fierce opposition to the Spanish government and the German Government. It declared that the day Prince Leopold of Hohenzollern was crowned King of Spain would be the day France declared war on Prussia and Spain.
The hegemon of Europe's threat was still quite effective. Under such pressure, Prussian King William I also showed some hesitation and even publicly stated that he did not support his cousin Prince Leopold inheriting the Spanish throne.
The Spanish government panicked somewhat as well. If their choice of king led to war with France, it would undoubtedly be a loss that outweighed any gain.
William I's concession greatly satisfied the French government, and it also relaxed its vigilance toward Prussia, which had only recently defeated Austria.
At the beginning of July, the ambassador from France to Prussia even sought an audience with William I and conveyed French Emperor Napoleon III's hope that William I could guarantee he would never again press the already withdrawn candidacy.
Such a demand was utterly rude, and it left the originally conciliatory William I somewhat displeased.
The french people could never have imagined that, even with William I taking the lead in backing down, there was someone in Germany even more unyielding, someone who had already begun preparing for war with France.
Somewhat angered by the ambassador from France's rudeness, William I sent an urgent telegram to his chancellor Bismarck that very day, hoping Bismarck could devise a reasonable solution to Prussia's crisis.
But William I had not expected that Bismarck's so-called solution to the crisis was to defeat France through war—in other words, to solve the problem itself.
After receiving William I's telegram, Bismarck showed no panic whatsoever. A faint smile even appeared on his face.
After asking Chief of Staff Moltke whether he was certain of victory in a war against France and receiving an affirmative answer, Bismarck decided not to wait any longer.
He drastically revised William I's telegram, deleting every word about careful deliberation and tactful concession. At the end, he added several remarks meant to provoke the French government and its officials: "Because of your country's unreasonable demands and attitude, His Majesty King William I has decided that from this day forth, he will refuse to receive the ambassador from France. He has informed your country solely through this telegram that there is nothing left for our two nations to discuss."
Although this telegram, intended to provoke the French government, was not sent directly to the French government's telegraph office, it was published as news in Prussia's major newspapers and spread throughout Europe through all Prussian diplomatic missions abroad.
The Prussians' resolute stance caused an uproar within the French government, thoroughly enraging the French government that had once been so smug.
Perhaps even Carlo had not expected that his tiny, subtle action would directly affect the world and bring the Franco-Prussian War forward by a full year.
On July 11, 1869, after several days of preparation, the furious French government expelled the Prussian ambassador to France and, without a trace of expression, announced the war between their nations to him.
Yet the french people did not see the panic and tension they had expected on the Prussian ambassador's face. They could only watch him leave the embassy with a relaxed expression and board the train back to Prussia.
Even more bewildered than the french people was Prussian King William I. Though William I was angered by the french people's rudeness, he had truly never intended to go to war with them at such a moment.
But with matters having reached this point, William I could no longer stop the war from proceeding. On July 12, William I declared general mobilization throughout the North German Confederation and called upon every state in the Confederation to fight for Germany.
Prussia called this war the Franco-German War, attempting to use the unity of Germany to demand that the various states contribute troops and strength.
The french people, meanwhile, called it the German-French War. The grievances between the french people and the German Region had not arisen overnight.
This war, larger in scale than the Austro-Prussian War, erupted with the speed of thunder before one could cover one's ears, catching the Spanish government—nominally the direct cause of the Franco-Prussian War—somewhat off guard.
However, with events having reached this stage, no matter who won the Franco-Prussian War, Prussia's Prince Leopold was no longer suitable to remain a candidate for King of Spain.
Furthermore, Prince Leopold himself publicly stated that he refused to inherit the Spanish throne. Although this was more because William I had stepped forward in opposition, Prince Leopold had ultimately given it up himself.
After all these events, only two of the original five candidates for the Spanish throne remained: the two princes of Italy's House of Savoy.
Whether Amadeo or Carlo became King of Spain, Savoy would be the ultimate victor.
Setting aside House of Savoy, an advantage they both shared, Amadeo's strength was that he was older and more mature and steady. Carlo's strengths were his friendly reputation, which might win greater recognition from the Spanish, and his greater likelihood of supporting reform.
From the perspective of reform, Primo undoubtedly supported Carlo becoming King of Spain. After all, although Primo supported the monarchy, he was a thoroughgoing Reformist.
Serrano, meanwhile, was more of a neutral figure between the Reformists and Conservatives, and could also be considered a monarchist from the aristocratic faction.
Neither man could make up his mind between Amadeo and Carlo. For the stability of the Kingdom of Spain, Primo decided to personally travel to Italy to assess the two princes and determine the final candidate for King of Spain during this visit.
Serrano agreed to Primo's proposal and stated that he would support either Amadeo or Carlo becoming King of Spain, and was willing to pledge loyalty to the new king.
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