For the majority of the 19th and 20th centuries, rulers in the European empires had an almost neurotic fear of a pan-European conflict breaking out once again; a 'world war' in the same vein as the Great War¹, and other conflicts in the Early Modern period. Whilst there have been some apparent close calls - the Crimean, Franco-Prussian, Alpine, and Mediterranean Wars being examples - no conflict has exploded to continental proportions since 1815.
My question is: how could you get another war in the same vein as the Great War to break out? There are many reasons why another major conflict hasn't developed in over two hundred years (interconnected economies, interconnected royal families, European cooperation in the undeveloped world, etc.), I don't believe these have always been enough to prevent another pan-European conflict.
So, what PODs are available to have a 'Second Great War' break out in Europe post-1815?
Note 1 - The 'Great War' is a historical term used ITTL to describe the OTL French Wars of 1792-1815.