There were many reasons that they thought it might be short and quick. They didn't think the economies could take that kind of strain for a long time, they thou

There were many reasons that they thought it might be short and quick. They didn't think the economies could take that kind of strain for a long time, they thought technology would make wars faster (British cavalry journals before the war thought the machine gun would help more than hinder the cavalry), and they just had too much faith in their first strike capability (France with plan XVII and Germany with Schlieffen plan). The biggest problem was that it had been over 40 years since Europe had been at war. This meant that there was little evidence based study to do with new military technology advances and it was too easy to dismiss colonial conflicts (Boer War, Russo-Japanese War) as not indicative of what a general war in Europe would look like. It is actually remarkable to hear some people argue that a world war today would be short (if it is non-nuclear) because of the same reasons.