I assume the OP aimes at the question, why the 'great powers' at the outbreak of WW I weren't prepared for a long war and not prepared for a 'total' war aka their industries and economies not prepared for what eas to come.
... and why everybody 'thought' it might be a short war ("back when the leaves fall" or "back at christmas").
True the wars of 'real' powers up to and including 1870/71 were 'short' wars, though the last was already thought to be unduley long.
But these wars were also still fought and conducted in a 'napoleonic' manner : columns of troops manouvering over countries to finally meet at some point. ... wihtout much involvement of the rest of the countries involved (beside the actual battlefield and directly marched though regions).
Sidenote :
Exactly that, that suddenly non-military-trained persons were going into action and involved themself after Gambetta took power was what deeply shocked and embarressed the prussians/germans and caused their 'Franc-tireur'-trauma, that led to the 'belgian atrocities' in 1914 as well as the unnecessary bombardement of Kalizs.
But from then onwards almost every military leader in every country saw the danger and anticipated, that the next 'Big War' would be a total as well as a long war, as they would (have to) include ALL of a belligerents resources - of manpower as well as of economy as well as of the soaring industry.
Even Falkenhayn said during the July-crisis, to an US-interviewer IIRC, that the war would last at least 3-4 years.
They also very well anticipated, what such a war would do to their societies :
social unrest, upheaval, revolution, an almost complete turn-over of their society.
There was a substantial and very present 'red scare' in the opening years of the 20th century. The 'Socialists' popping up and growing tremendously fast, as fast as the industry.
In retrospect : they were complety right. No society emerged the WW I without heavy and hurting changes.
Therefore, the wish, to avoid exactly this gave birth to the 'short war' myth, that embraced virtually every participating power in 1914.
It could not be, what must not be.
Ofc the industry well agreed, that such a war (long one) would be desatrous to their emerging global economy.
And in that only underpinned, that a long war had to be avoided at all costs.
Therefore, the wishfull thinking, that a short war was/had to be not only short, but would/had to be also successfull gained so much atractivity opposite to the realistic and well anticipated outlook of a total war, which would leave the winner in an almost as bad condition as the looser, that it developed so much propagandistic momentum to brush away any thoughts for preparing for the 'real' stuff.