Austria collapses into a radical workers state along the Danube in 1848. Russia tries to intervene through the Pannonian Plain and is soundly trounced, leading to the Danubian principalities being absorbed by the state, while Italy gets... complicated, in a religious/ethnic conflict way. Eventually the Danubians are mostly successful on the peninsula, with Lombardy, Venetia and the Papal Legations merged into the greater state, while the rest of Italy unites under an aggressive papacy protected by the Savoyards in the north and the Bourbons in the south, under French tutelage.
In the meantime France is stabilized as a moderate republic, with Russia undergoing earlier reforms in the face of extensional destruction, while Prussia gathers the remaining German states under her wing who fear Vienna, except for a few states that can't decide if they hate the radicals in Vienna or the reactionaries in Berlin more.
Over the next twenty years the Danubian worker's state expands into the Balkans as Vienna stirs up ethnic and religious strife to break away the frontiers of the Turk's empire.
By the late 1870s early 1880s the next great European conflict breaks out when the Danubians try to bring Bavaria into their sphere of influence and Paris, Berlin, St. Petersburg, (as well as Istanbul and Rome) move to crush the worker's state.