G.a. Henty 
The Lion of the North 
A Tale of the Times of Gustavus Adolphus

The Thirty Years' War arose from the determination of the Emperor of Austria to crush out Protestantism throughout Germany. Since the invasion of the Huns no struggle which has taken place in Europe has approached this in the obstinacy of the fighting and the terrible sufferings which the war inflicted upon the people at large. During these thirty years the population of Germany decreased by nearly a third, and in some of the states half the towns and two-thirds of the villages absolutely disappeared…