13. The Thirty Years War, 1618-1648 3-11.5 million
A vast religious war in the Holy Roman Empire, it was waged by Germany against most of Europe at one time or another. You think we've been in Afghanistan for a long time? Try 30 years of constant conflict. Primarily fought between Protestants and Catholics, it eventually turned massively political, and was the basis of most of the power struggles in Europe for decades to come. The aftermath of the war was the destruction of massive areas of farmland. Not just through battle, but because the armies would devour anything they came across, destroying crops as effectively as any locust.