Following two utter devastation of over two decades of conflict, first in the French Revolutionary and then the Napoleonic Wars, the Great Powers of Europe came together at the Congress of Vienna with the intentions of creating a permanent peace that would prevent the occurrence of a similar continent-wide period of blood-letting. Following months of negotiation, the resulting post-war settlement would lead to the establishment of the "Concert of Europe", a fragile balance of power structured in such a way so as to make the rise of another continental hegemon impossible, as well as the Congress system - arguably the first ever attempt to formalize a multilateral forum of international diplomacy.
The Congress system did not last long - less than a decade after its establishment, the Great Powers were once again at each other's throat, feuding over both of their spheres of interest as well as the fundamental ideological disagreements between the liberal occident and conservative orient. However, the Concert of Europe, for as much as it failed its ambition in preventing war altogether, nevertheless allowed the achievement of the "Long Peace", almost a century wherein conflicts were typically localized and fought between two belligerents without dragging in the entire continent, with its peaceful symphony being largely uninterrupted barring the revolutionary waves of 1848 and the 1856 Crimean War.
Although every student of the past knows that it is folly to engage in historic determinism, it has become an oft repeated truism that the transformation of Germany and Italy - two of Europe's traditional battlefields - from a fractured collection of statelets into unified polities irreversibly upset the continental balance of power. The year of 1872 can be remembered as the day that the Concert of Europe, having already been in terminal decline, would come to an ignomonious end, and the start of a decades long period of international polarization that would explode in 1914 into the conflagration that eventually became known as the Great War.
By the 1860s, the German Question dominated the politics of the members states of the Confederation. Despite the subduing of the liberal nationalist project advocated by the 1848 Revolutionaries, nationalist sentiments remained high. Ever since the dissolution of the Holy Roman Empire after the end of the Napoleonic Wars, the various duchies, kingdoms and principalities of Germany operated under the loose "German Confederation", the leadership of which was increasingly contested between the southern Austrian Empire and northern Kingdom of Prussia. More than mere geopolitical rivalry, each state presented a different ideological vision to the issue of unification - either a Greater Germany encompassing all Germans dominated by the traditional Catholic Austria, or a Lesser Germany dominated by the modernist Protestant Prussia. Austria however, did not wish to give up its vast multi-ethnic domains as would be required to lead Germany. Hence, Prussia would rise to the occasion.
In a 1862 speech, Minister President of Prussia Otto von Bismarck would declare that the unification of Germany would be achieved by Prussia's Armies with "Iron and Blood". In 1864, the first steps would be taken in the Second Schleswig War, where Prussia and Austria would achieve a decisive victory against Denmark, conquering the provinces of Schleswig and Holstein. Then in 1866, taking advantage of Italian and Austrian tensions, Prussia would inflict a crushing victory against its former ally, formally dissolving the German Confederation and replacing it with the North German Federation - a Prussian federal union that would serve as the foundation of a future unified state. All that was left to conclude unification was to bring Austria's former allies, the remaining southern catholic states of Bavaria, Württemberg, Baden and Hesse, under Prussia's dominion. However, the Austro-Prussian War had created deep resentments, the overcoming of which required an external enemy against whom the remaining German states could be united against.
The Second French Empire had long eyed Prussia's ascendancy with suspicion, particularly as a unified German could present a credible challenger to France's status as the pre-eminent continental land power of Europe. Having created a semi-liberal regime in the image of his uncle, Napoleon III's government had been recently undermined by several recent foreign failures, most notably the overthrow of its puppet, the Second Mexican Empire. With both Republican and pro-Bourbon sentiment, like many unstable autocracies in history, France sought to recover its prestige and stability with a successful war. Thus, for Bismarck, France was the perfect target for provocation. Through years of successful diplomatic efforts and taking advantage of French aggressiveness, Bismarck was able to alienate France from the other Great Powers and leave it wholly isolated. Now, all that was left for Prussia was to goad France into issuing a declaration of war, the opportunity for which would arrive with the Hohenzollern crisis.
While the decline of the Spanish Empire had started in the 17th century, Napoleon's destructive Peninsular campaign devastated the Spanish metropole, crippling its hold over its American possessions, causing their secession during the Latin-American Wars of independence, depriving the Kingdom of Spain from the extracted resources and colonial revenue it had relied upon. Spain would spiral into a vicious cycle of political infighting between conservative and progressive forces, experiencing almost a dozen coups and three "Carlist" civil wars regarding disputes over dynastic succession between the Isabella II and Carlos V, the daughter and brother of former king Ferdinand VII respectively. While the accession of Isabella was supported by Spanish liberals who hoped she would differ from the conservative absolutism of her father, as her rule progressed, she had increasingly succumbed to traditionalist urges, eventually leading to her being ousted in the 1868 Glorious Revolution by General Juan Prim, the leader of the Progressive Party.
The deposition of Queen Isabella II resulted in a two-year period of interregnum, as the Spanish legislature, the Cortes General, asserted itself an acceptable new monarch, one who was simultaneously willing to be elected by the Cortes General, would swear to uphold and abide by the newly written liberal democratic constitution that reduced the crown to a ceremonial role, and one who would be acceptable to both the Spanish people and political establishment. With no viable candidate found in Spain (Carlos being too absolutist, Isabella's son Alfonso too easily influenced by his mother, whilst other noble families of sufficient status in Spain had too many enemies), the search was expanded to the rest of Europe's ruling dynasties. Eventually, two acceptable candidates emerged: Amedio di Savoia, the second son of Italy's King Victor Emmanuel II, and Leopold, Prince of Hohenzollern, a distant cousin to the ruling Hohenzollern dynasty in Prussia. However, both were wary at the offer, Amedio feared the instability of Spanish politics may eventually lead to his death, while Leopold feared inciting conflict with France.
It was this latter reason that brought Bismarck's attention to the matter when Leopold informed Prussian King William III of the Spanish ambassador's offer. The election of not merely a German prince, but a relative of Prussian royalty, to the Spanish throne would stoke French fears of geopolitical encirclement and potentially provoke a hostilities, providing the necessary pretext for war. Furthermore, in such a hypothetical war, Spain would be compelled to assist, if not militarily, then diplomatically and further isolating France. Bismarck would thus place significant pressure on Leopold and William III to accept the offer, while having Prussian agents work with the pro-Leopold faction in Spain to run interference, making it seem that Amedio was the likely pick. This interference was highly successful, as the French government would only find about Leopold's candidacy the morning after he had been formally elected as Spain's future king by the Cortes General, in an extraordinary session held on the 8th of July.
France's reaction to this news was apoplectic. Both the governments of Prussia and Spain found themselves confronted by furious French ambassadors, denouncing this "trickery" and demanding that Leopold's candidacy be withdraw at once - or else. Further, the ambassador demanded a guarantee from King William III that not only he block Leopold's ascension to the Spanish throne, but that he also promise to never put forward a Hohenzollern candidate for the Spanish throne in perpetuity, demands which were politely rebuffed by the King. Seizing the opportunity, Bismarck would leak the report of this interaction to the public, intentionally edited in such a manner to make it seem like both the King and the ambassador treated each other incredibly rudely in violation of expected diplomatic decorum. This "Ems Dispatch" would achieve its inflammatory effect, outraging both the French and German publics. Like a raging bull seeing a red rag, the French Corps législatif voted declare war on Prussia, with a Spanish intervention tabled after Prussia's defeat.
Bismarck was a man with a plan, and he had finally achieved what he wanted.
Authors Notes:
Here, we have our first chronological POD. From what I've read, Bismarck's concern for Spain's vacant throne was his ability to use it as a way to goad France into war. Here, his attitude is more "Holy, I can have a war with France AND a Hohenzollern on the Spanish throne", which leads to him taking more decisive steps to secure Leopold's accession. Given the ceremonial nature of the crown under Spain's new constitution, as well as residual sympathy from Spanish liberals about Napoleon III's more "progressive" regime (though this sympathy is souring fast due to France's interventionism into Spain's internal affairs), Leopold's accession is not enough to have Spain directly intervene in the Franco-Prussian War (not yet at least).
How these close dynastic ties between Germany and Spain will impact the future Spanish-American War is another question all together...
