You can say that it really began on June 28th, 1914 and escalated from there, but we're almost at that time. Only a handful people remember the world as it was being torn by the Great War.
I hope we never have another Great War. The fact our species has had 2 world wars (technically it's had as many as two dozen, but that's another story) resulting in such massive global casualties is a shame. Perhaps inevitable for a species in the throes of first developing mass-scale technology, but shameful nonetheless.