• [Did any WWI Commanders study the American Civil War before the war?] (http://www.reddit.com/r/AskHistorians/comments/34w68v/did_any_of_the_world_war_one_commanders_study_the/)

• [Was a 'quick war' the prevailing belief before 1914?] (https://www.reddit.com/r/AskHistorians/comments/31iteb/the_war_will_short_and_casualties_low_was_this/)

These are answers I've given before on the subject.

While the idea of a 'short war' seems to have been generally accepted, the testimonies of numerous statesmen and generals from the capitals during the July Crisis indicates that serious doubts were held. There is no evidence whatsoever that the saying 'the war would be over by Christmas' was ever widespread in 1914; the closest is the Kaiser's assurances that the Western Front armies would be home 'before the leaves fell'. When the Tsar signed the orders for Russian mobilization, he informed his Generals that he knew was signing orders that would 'lead to the deaths of thousands of men'. In Britain, Kitchener called up his volunteers with the understanding that they would not be ready before 1915, and that Britain needed to be ready for a long war. Of course, there were the writings of Norman Angell and Jan Bloch, both of whom painted very dim pictures of the conduct of future wars.

As Holger Herwig points out in the article Germany and the "Short War Illusion" (available on JSTOR), everyone from Ludendorff and Falkenhayn, to Moltke and Tirpitz, accepted the likelihood that a future war would be a "People's War", and would be a long and bloody struggle; the Schlieffen Plan was, unfortunately, the only plan Germany possessed after 1913, and so was hoped to at least place Germany in a favourable position once hostilities broke out if it proved unable to vanquish the French Army in 40 days.

Some examples:

As late as 19 February and 14 May 1914, Moltke reiterated his fears that the future war would not mirror Schlieffen's blitzkrieg model. The chief of the General Staff informed first the Prussian minister of war and then the Prussian minister of the interior that the next war would "under certain circumstances be a long, drawn-out war." He specifically reminded General Erich von Falkenhayn, head of the Prussian War Ministry: "Our next European war will be a peoples' war in the fullest sense of the term." When the Prussian Ministry of the Interior turned its attention to the question of feeding the capital in case of war on 27 July 1914, Colonel Gerhard Tappen, Ludendorffs successor as head of the Mobilization Section, allowed that at the General Staff one planned on a war of "at most two years." Falkenhayn, for his part, early in August took leave of his old regiment, the 4th Foot Guards, with the statement that the war would last "at least one-and-one-half years." At Munich, Major Karl Haushofer, the future "father" of geopolitics, informed his wife that he expected the conflict to rage "at least 3 years,"

Economic planning even took into account a long war:

In June 1906-that is, just months after the Schlieffen plan had been finalized-the Prussian Statistical Office turned its attention to feeding the nation in time of war. It based its sobering analysis on calculations for two war scenarios: the first for a campaign of "3/4 to 1 year duration"; the second for "a war of 2 or even 2 1/2 to 3 years." Later that year, the Imperial Navy Office questioned the very validity of planning for a short war: "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 War Ministry, for its part, in 1894, 1906, and 1911 based its provisioning tables on a war of nine months' duration. Nowhere is there a discussion concerning a war of forty days.

Finally:

The General Staffs Third Section (Intelligence) in May 1910 warned Moltke not to expect to defeat the French armies either quickly or completely. Schlicffcn's proposed "first strike" would fall far short of destroying one million poilus in forty days! Two years later, Moltke as well as Colonel Ludendorff, then head of the General Staffs Second Section (Mobilization), bluntly stated that the German Army lacked the firepower to deal its enemies a knockout punch. Rather, Ludendorff in November 1912 spoke of the future war as "a long-drawn out campaign with numerous difficult, long-lasting battles, before we can defeat [even] one of our adversaries." He was shocked to discover that existing ammunition stocks sufficed for a war of only forty days.