> The experience of the long 19th century was that wars would be short, decisive, localized in scope, and fairly bloodless.

At the risk of writing a 1,000 word reply...for the past 2 decades there's been a growing body of academic research using archival documents from military archives (captured by the Soviet Union after WW2 and unknown to scholars until the 1990s), that politicians and generals didn't actually believe in the "short-war illusion" and they were actually fully prepared for the long, bloody stalemate that happened.

Christopher Clark's The Sleepwalkers is a popular book containing much of this new research.

Some examples from this kind of research:

Prussian General & Chancellor von Caprivi warned that war against France alone would be "tenacious and protracted."

Field Marshal von der Goltz publicly warned that "future wars would be long drawn-out affairs involving nations and not just cabinets."

In 1890 the architect of the Franco-Prussian War warned parliament that caution the nation that the next European conflict would most likely be a "Seven or Thirty Years' War."

The quartermaster-general of the German army warned "We cannot expect quick, decisive victories." The war of the future instead would feature "a tedious and bloody crawling forward step-by-step," that is, "siege-style" warfare.

Field Marshall von Haenseler also warned that one could not simply "carry off the armed forces of a great power [France] like a cat in a sack."

The German General Chief of Staff believed the coming war "will be a peoples' war, one which would not be concluded by a single decisive battle," but which would deteriorate into a "long and protracted struggle." The future war could not be terminated until "the peoples' energy had been entirely broken"; even if victorious, the German people would emerge from such a struggle "exhausted in the extreme."

The German Imperial Navy office wrote "It is not possible to discern why the war should last only nine months.... In order to avoid arriving at deceptive perceptions, one should, on the basis of available information, base planning on a war of perhaps 1 1/2 years."

The Prussian Statistics Office (who had to figure out how to feed the nation during the coming war) calculated a scenario for "a war of 2 or even 2 1/2 to 3 years."

In 1912 Field Marshall Ludendorff wrote any war would be "a long-drawn out campaign with numerous difficult, long-lasting battles, before we can defeat [even] one of our adversaries."

The General Staff's Third Section (Intelligence) continued to speak of the war of the future in terms of a "peoples' war," of a "protracted struggle," and of a "long-lasting campaign."

German Major Haushofer (the future "father" of geopolitics) believed the coming war would last "at least 3 years".

Falkenhayn told his old regiment the coming war would last "at least one-and-one-half years."

German Chancellor Hollweg believed that any action against Serbia "can lead to a world war".

According to this line of research, the short-term war illusion still existed but in a different (worse) guise. The generals and politicans knew (or strongly believed) the coming war would be anything but short, decisive, localised, and bloodless. But they couldn't see any alternative.

They realised that there was no viable alternative to a desperate HOPE of a short war, short of admitting that war itself was no longer feasible.