The revolution in Spain and the establishment of a Provisional Government attracted the attention of many countries in Europe.
After the Spanish Parliament announced that it would not abolish the Monarchy and would seek a new King, many countries were curious and watched with the mindset of spectators, wondering which lucky person would become the new King of Spain.
But soon, the French Government could no longer laugh.
According to news from Spain, among the many candidates for King of Spain, Prince Leopold of the Hohenzollern Family ranked highly and was very likely to become the next King of Spain.
(There were many Prince Leopolds in the German Region, which translates to Leopold. The candidate here is Prince Leopold of the Hohenzollern Family, not the one from Bavaria. Prince Leopold of the Hohenzollern Family is the father of Ferdinand I, King of Romania.)
This was deadly for the French. A single Prussia was already a huge threat to France. Especially after defeating Austria, the threat Prussia posed to France was unprecedented.
If someone from the Hohenzollern Family were to obtain the Spanish crown, France would face the predicament of being simultaneously attacked by Prussia and Spain.
Although Spain couldn't be considered a strong power and was barely a second-rate country in Europe, it was located in southern France and had the Pyrenees Mountains, a natural defensive line.
Prussia would only need to stockpile less than 100,000 troops in Spain to create significant trouble for the French and threaten many cities in southern France.
On June 21, 1869, the furious French Government strongly opposed the Spanish and German Governments, declaring that if Prince Leopold of the Hohenzollern Family were crowned King of Spain, it would be the moment when France declared war on Prussia and Spain.
The threat of a European hegemon was quite effective, causing William I, King of Prussia, to hesitate and even publicly state his disapproval of his cousin Prince Leopold inheriting the Spanish throne.
The Spanish Government was also a bit panicked, as having their choice of King lead to a war with France would undoubtedly be a costly mistake.
William I's concession satisfied the French Government greatly, easing their vigilance against Prussia, which had just defeated Austria.
The French Ambassador to Prussia even sought a meeting with William I in early July, conveying the hope of French Emperor Napoleon III that William I would guarantee not to restore the previously abandoned candidacy.
Such a demand was certainly rude, causing dissatisfaction in William I, who had initially intended to concede.
The French could not have imagined that, even when William I was making the first retreat, there were still more stubborn individuals in Germany, who were even preparing for war with France in advance.
William I, irritated by the French Ambassador's insolence, sent an urgent telegram to his Prime Minister Bismarck on the same day, hoping Bismarck could devise a reasonable plan to resolve the crisis Prussia faced.
However, William I did not expect that Bismarck's so-called solution to the crisis was to defeat France through war, thereby resolving the issue itself.
After receiving William I's telegram, Bismarck showed no panic and even displayed a slight smile.
Upon asking Chief of Staff Moltke if they were assured of victory in a war against France and receiving a positive answer, Bismarck decided not to wait.
He drastically altered William I's telegram, deleting all the remarks about taking a long-term view and mild concessions, and added provocative words against the French Government and officials at the end of the telegram: "Due to Your country's unreasonable demands and attitude, His Majesty King William I has decided to renounce proximity to the French Ambassador from now on and to only communicate through this telegram, as there is nothing left to discuss between our two countries."
Although the telegram meant to provoke the French Government was not directly sent to the French Government's radio, it was published openly in major Prussian newspapers and disseminated throughout Europe through all of Prussia's foreign missions.
Prussia's determined attitude stirred up an uproar in the French Government, frustrating an otherwise triumphant French Government completely.
Perhaps Carlo did not expect that a subtle gesture from him would directly influence the world and lead to an early outbreak of the Franco-Prussian War by a year.
On July 11, 1869, after several days of preparation, the angry French Government expelled the Prussian Ambassador to France and coldly announced the war between the two countries to the Prussian Ambassador in France.
However, the French failed to see the expected panic and tension on the face of the Prussian Ambassador, instead watching him leave the embassy with an air of ease to board the train back to Prussia.
More baffled than the French was King William I of Prussia. Although William I was angry at the rudeness of the French, he truly did not intend to go to war with the French at this moment.
But with things reaching such a point, William I could not prevent the war from proceeding. On July 12, William I announced a general mobilization in the Northern German Federation, calling on all states within the Federation to fight for Germany.
Prussia named the war the Franco-German War, attempting to request the states to contribute troops and resources in the name of a united Germany.
The French named the war the Franco-German War as well, as the grievances between France and the German Region were longstanding.
This war, on a scale larger than the Austro-Prussian War, erupted with surprising swiftness, unexpectedly involving the Spanish Government, theoretically the immediate cause of the Franco-Prussian War.
Nonetheless, given the circumstances, regardless of who won the Franco-Prussian War, Prince Leopold of Prussia no longer seemed suitable to become King of Spain.
Moreover, Prince Leopold himself publicly announced his refusal to inherit the Spanish throne, although primarily due to William I's opposition, but it was still Prince Leopold himself who relinquished it.
After such events, the original five candidates for the Spanish throne were narrowed down to two princes from the Italian Savoy Dynasty.
Whether Amadeo or Carlo became King of Spain, Savoy would be the ultimate winner.
Putting aside the shared advantage of the Savoy Dynasty, Amadeo's advantage was being older and more mature and steady. Carlo's advantage was possibly having a reputation more recognized by the Spaniards for friendliness, and more likely to support reform.
From the perspective of reform, Prim undoubtedly favored Carlo as King of Spain. After all, while Prim supported the Monarchy, he was a thorough reformist.
Serrano leaned more towards a neutral stance between reformists and conservatives and could be considered part of the Royalist Faction of the nobility.
Both were undecided between Amadeo and Carlo. For the stability of the Kingdom of Spain, Prim decided to personally visit Italy to evaluate the two princes and determine the final candidate for King of Spain during this visit.
Serrano agreed with Prim's proposal and expressed his willingness to support either Amadeo or Carlo as King of Spain and to pledge allegiance to the new King.