Inspired by recent WWI related threads:
As Barbara Tuchman and many others have noted, European politicians and generals almost universally believed that the "Great War" would be over in a matter of weeks, or by Christmas at the latest. With hindsight we know that was never very likely, due to advances in technology, and other factors. Nevertheless, such a short war would not have been impossible.
The challenge, should you choose to accept it, is to come up with a plausible scenario in which the Great War ends in a ceasefire no later than Christmas 1914. The POD should be no later than July 28, 1914 for international events (i.e. the assassination of Franz Ferdinand and the declarations of war by the major combatants), which should be taken as exogenous.