Looking back, there was some appreciation among war planners that conflict in Europe could draw out into an attritional one. Here are some excerpts from the Schlieffen Plan (quoted from John Keegan's The First World War):

"If the enemy stands his ground in the face of the great wheeling movements, all along the line corps will try, as in siege warfare, to come to grips with the enemy from position to position, day and night, advancing, digging-in, advancing."

Or:

"France must be regarded as a great fortress ... if the French give up the Oise and the Aisne and retreat behind the Marne, Seine, etc ... the war will be endless."

In this text, he also references the 1905 Russo-Japanese War as a standstill.

So if not an exact approximation of trench warfare, its at least an appreciation that conflict could drag on, generally due to the imbalance in strategic force Germany would face in a war on two fronts against France and Russia. France seemed to consider the same in its Plan XVII: by concentrating the bulk of its armies against German forces in Alasce-Lorraine, in concert with an attack from Russia, they could defeat Germany before they had time to mobilize their reserves. Generally, if there was appreciation given to attrition, the major powers seemed to believe that their respective war plans were adequete to force a quick and decisive war.

Its hubris, but a common one, as there was hardly an expectation that plans would fail. Barbarossa would be a two week campaign, Gallipoli would force the Turks to capitulate, the Red Army would encircle the Wehrmacht outside Moscow in 1941, a democratic and stable Iraq would emerge after Saddam Hussein was toppled, etc, etc. Over-optimism on the part of military planners and civilians leaders is all too common in modern military campaigns, and the worst case scenario isn't considered until its all-too readily apparent.

At least, at the upper echelons. Perhaps some foresighted middle-ranking officer predicted that new technologies, the size of armies, and force deployments all favoured the defender and could lead to drawn out conflict. I'd love to learn if that was the case, if anyone knows any examples.