Point of divergence: Napoleon III isn’t captured during the Franco-Prussian war.
This war is still lost for France, but the territorial loss are less important than in OTL and the Empire survives.
However the defeat has increased the internal tensions in France and when Napoleon III dies in 1873, the Empire is overthrown by a republican revolution which derives quickly to something close to the OTL “Commune”, but with much more importance.
While the frightened moderate republicans are now backing the young but determined Napoleon IV, diving France into civil war, the revolutionary movement is spreading over Europe, causing a new Spring of Nations. Only the recently unified Germany and Italy aren’t affected at this time. Besides, the first is preparing the incoming war with the republican France by sending ultimatums to its western neighbors (without available allies to help them, the Netherlands agrees to cede the Limburg and the Luxembourg, and Belgium to cede Arlon), while the second sizes the occasion to declare immediately war to France to get Rome, Nice, and possibly Corsica.
In Great Britain, the monarchy surprisingly falls. In Russia, Spain and Portugal too. The Ottoman Empire and Austria-Hungary are disintegrating.
For the Germans, it’s a unique opportunity to expand to the East, and they declare war to Russia, officially to restore the monarchy. Austria is pacifically annexed after a plebiscite (Italy getting the Trentino), but Germany now wants to succeed to the Austrian Empire in the region by vassalizing the Danubian states, and starts to occupy them with a “pacification” goal.
They are indeed in state of war: Each Slavic neighbor of Hungary wants its part of this multi-ethnic state. Moreover, the new Bulgarian Empire, born on the ashes of the Ottoman Empire and massively supplied by Germany and Italy has conquered most of the Balkans and now contribute to unbalance the situation in the region.
The move of Bismarck toward the East is initially a success, but will prove to be too ambitious.
In France the war against Italy speeds up the end of the civil war: the supposed pacifism of the French revolutionaries and the military successes of the “imperial” army against the invader achieve to reunite France behind Napoleon IV. It’s the occasion for him to remake its relatives’ campaigns of Italy and to show its legitimacy by military victories and annexations. Italy has now no chance of victory. The fast progression of the French army causes the end of the Kingdom of Italy. As a now revolutionary State, Italy is joined in its defensive war by Spain, which helped the French revolutionaries before. What we call the Latin War, will be short and victorious for the imperialist powers. France is helped in its war by the Empire of Brazil, which tries to restore the monarchy in Portugal, where the revolution was particularly bloody.
After the victory, Pedro II decides to make a political union with the Brazilian mother country, an economically beneficial one for Portugal, but where Brazil will be the mainland.
Napoleon IV, instead of creating allied regimes in the defeated and unstable countries (which also means to have to defend them), decides to sell the peace at a high annexation cost, for prestige reasons, and to strengthen France against Germany.
For the same reasons, and because Germany is now too busy with its war with Russia and many uprisings in the Danubian territories, he decides to invade Belgium (officially to pacify the country). As the Netherlands react by blocking the port of Antwerp, Napoleon IV declares war to them.
Great Britain, as for the German ultimatums, can’t react, as the British Empire is collapsing: The overthrowing of the monarchy has led most of its colonies to declare their independence. The only way for the new regime to keep its influence in the world is by trying to place friend regimes in its former colonies. It will work for some of them (like India), but not for all (like South Africa). With Canada defeated by the United States, most of what remains of the country will be made of French speakers, who will later ask for the protection of the French Empire.
These 4 years series of local wars we now call the “Winter of Empires” end with the defeat of the German empire against Russia (now Eurasia, to better reflects its minorities) and its allies. The German internal tensions due to the war duration, The alliance of the Danubian clandestine governments, the military reorganization of Eurasia, and the British supplies, have reversed the war balance and put Germany in an untenable situation. Bismarck asks for an honorable peace to save the Empire. Some territories are lost, where Germans are a minority or to strengthen the position of Poland or the new Danubian Confederation. But with most of Austria annexed, Germany is still greater than before the war.
In the post-revolution years, the world will divide in two blocks: one the revolutionaries call the “Imperialist States” and “People’s States”, made of the countries where the revolution has succeeded. At start, these peoples weren’t uprising for the same reasons. For example, the UK revolution had mainly social reasons. In Spain, it was more political. While they were clearly nationalists in Austria-Hungary. But with the time and because of their military alliance, their respective regime will tend to conform to the British-Eurasian vision of the revolution: an authoritarian socialist and internationalist regime.