The 1848 revolutions result in a brutal war of German unification, with either a republic or a Protestant state winning. The new Germany launches a kulturkampf against Catholics in the German Reich. Just like the French revolution, this strikes fear into the remaining conservative and Catholic monarchs.
As a result, a *fascist Roman League between the Papal States, the Hapsburgs, Emperor Napoleon III, and the Bourbons of Spain is formed.
A shrewd and charismatic political manipulator then worms his way into power, blessed by the Pope himself and commanding an army of fanatic supporters. Said charismatic leader then pledges to eradicate the Protestant heresy once and for all. Germany, the Netherlands, Britain, the Ottomans, and Russia form an alliance of convenience in response.
However, said manipulator surrounded himself with sycophants and yes-men. When the war begins, the Roman League makes astonishing gains yet commit unspeakable genocides. These gains are eventually reversed with the German flag flying over St. Peter's Basilica. Some Protestant extremists call for the Basilica to be blown up, but cooler heads prevail.