More can be written, but you might like to start with 'Is there any evidence AT ALL that anyone involved in the early stages of WW1 thought the war would be "over by Christmas"?' by /u/DuxBelisarius. I don't see the relevance of the links at the top other than the last two -- the few I checked are general discussions of World War I, not addressing this -- but the main body and replies do answer the question. The next to the bottom link, pointing to "Was there any way that Europeans in 1914 could have predicted that WWI would be more like the American Civil War (in terms of scale, length, and severity) than the Franco-Prussian War, which was more recent (1870 vs 1865), on the European continent and involved the same powers?", has more information, like a pointer to an article on JSTOR.

This is not to discourage discussion. More questions, data and debate are welcome.