I typed "trench" into Google Ngram viewer, and got the following results: http://i.imgur.com/tyrEsHH.jpg - why the sharp increase at this time? Was it commentary on the Second Boer War, for example?
Well, the answer is that it didn't. If you zoom in and set the parameters just from 1900 to 1930, the rise isn't in 1907. Zoom in even more so we see year by year, and the mentions remain steady past 1913, 1914 that you see the major increase begin, which can clearly be ascribed to the First World War.
