The Continental War of 1914-1916 reoriented Europe and established Germany and Austria as the preeminent powers.
France was defeated and force to cede Belfort, Briey-Longwy, and all of Lorraine east of the Moselle River to Germany.
In the east, Russia was forced to cede Congress Poland, Lithuania, Courland, Kars, Batumi, and Yerevan.
Serbia was vassalized, Montenegro annexed, Bulgaria expanded, and Austria satisfied.
It was messy, but the social cost was relatively limited compare to, say, the upheavals of 1848 or the Napoleonic Wars.
What if the war had been more drawn out and socially disruptive?