The First World War
Polish railroad pioneer Jan Gotlib Bloch was probably the first person to truly understand what the evolution of military technology and geopolitics meant for the future of warfare. In 1898, he published a series of 6 books that talked about the transformation of modern warfare. He predicted that the next war fought in Europe would rely on trenches.
This next war would kill millions, result in famine and epidemics and would create a situation so horrible that it would lead to more war. Sixteen years later, WWI was declared, and confirmed all of his predictions.