People should have known in 1914 when the guns of August began firing that this was going to be a serious, bloody affair. The seven years between 1898 and 1905 had lessons aplenty to suggest that the butcher's bill would be higher than anyone was expecting. A series of conflicts where a major power confronted a minor one highlighted the new technologies that would play such a major role in the World War.
All of these should have been wake up calls. In each case, what should have been an easy win over weak opponents came either at ruinous cost, or was a humiliating defeat. The technologies were easily usable even by these opponents. Planners should have extrapolated what would be the result when the Mausers were wielded not by poorly fed Boer farmers, but by the Imperial German Army.
And yet Europe was eager for War in 1914. People hadn't been paying attention. They would soon.