At first the Protestant princes were slow to join forces with Gustavus. This was because they dreaded the vengeance of Ferdinand, and feared to be assailed by Tilly, who was now besieging the important town of Magdeburg, in Saxony. Magdeburg had refused to obey the Emperor's decree against the Protestant form of worship, and the cruel Tilly was resolved to seize the town and give it up to his soldiers to sack. The citizens made a splendid defence and held Tilly out for six months. But, unluckily, before Gustavus could come to their aid, the town was seized by a sudden assault in May 1631. A terrible scene of butchery and plunder followed. The ferocious troops slew all before them, men, women, and children, and their savage old general made no attempt to check them. Fire and pillage laid the town in ruins, and the fate of Magdeburg sent a thrill of horror through the whole land.

But the Lion of the North was already preparing to spring upon Tilly. Gustavus, with a number of Protestant princes who had at last joined him, marched upon the Emperor's army, and faced it at Leipzig in September 1631. He won a complete victory. Tilly was now seventy-two years of age. He had fought many battles, yet had never been wounded. At Leipzig he had to fly, his body pierced by three musket balls. His troops fell in great numbers, and when they broke and ran, the peasantry avenged their wrongs on the soldiery who had ill-treated them by slaying the fugitives in hundreds.

Gustavus marched through Germany in triumph and was received everywhere with great joy. But Tilly gathered another army and again he and Gustavus met at Rain, on the Lech. In this battle Tilly received a mortal wound and died in a few days. His death left the Emperor without a great commander, for Wallenstein had been dismissed: he had offended the Catholic princes of the League by his overbearing conduct, and Ferdinand had been forced to part with him.

The Emperor now turned to Wallenstein and asked his assistance: the latter would only give it on his own terms, to which Ferdinand was compelled to agree. Wallenstein took the field and, in November 1632, found himself face to face with the great Gustavus Adolphus on the plain of Lutzen. Here a great battle was fought, and the Swedes won a victory for which they paid a terrible price. For their mighty leader and king was killed almost as soon as the battle began, and his body was found beneath a heap of slain.

The death of the great Protestant hero marked a dreadful change in the course of the Thirty Years' War. The destruction and misery had been great before his death: it became far worse afterwards. Instead of a religious war pure and simple it became a war in which every leader fought for his own hand, every prince seeking to extend his own territories, every victorious army seizing the chance to despoil the unhappy people among whom they fought. When the firm hand of Gustavus was withdrawn, the Swedes became as wild and lawless as the rest, and poor, bleeding Germany was torn and devastated by the armies which marched across her hills and plains, the path of every destroying horde being marked by blazing towns and villages, and heaps of slaughtered country folk.

Wallenstein did not live long after the death of his famous opponent. The great captain of fortune thought to better himself by changing sides and fighting against the Emperor. But before he could do this his plans were found out, and he was murdered by a party of the Emperor's men in 1634. The Emperor's son, whose name also was Ferdinand, became commander of the Imperial armies and the war went on.

The Emperor died in 1637, leaving a name stained with blood. Few men have acted more cruelly than he. In order to force his Protestant subjects to profess the same form of religion as himself, he sent fire and sword through his native land, causing the death of millions of human beings, and laying waste vast stretches of fruitful country. He was followed by his son, Ferdinand III (1637-1657), and the war dragged on. For another eleven years Swedes, French, and Germans marched and counter-marched, fought, beleaguered towns, and carried the worst horrors of war to every corner of the land. At last the struggle ended in 1648: it ended in Prague, where it had begun, and it was closed by the Peace of Westphalia.

The war had been destructive to Germany: the peace was no less harmful. By its provisions France seized a portion of the Empire on the west, Sweden a portion on the north, Switzerland was separated from Germany, and the Netherlands became independent of Spain. But one of the most dangerous provisions was that which made every German prince or princelet an independent ruler in his own kingdom, and allowed him to form alliances with other states or with foreign powers. This tended to split up the Empire into a multitude of small isolated states, often warring with each other, rarely moved by the spirit of national unity.

National unity, indeed, had been almost destroyed by the frightful effects of the Thirty Year's War. Wealth and prosperity had fled. Arts and commerce seemed to be driven from the land. Prosperous towns, busy villages, homesteads, and cottages had become heaps of ruins. There were great numbers of small towns and hamlets of which barely the memory was left: the names were in the records, but no man lived who could point out where they had stood. In a great plain which had been filled with cornfields and orchards, with meadows and vineyards, the traveller could journey for many miles and see not a single house, not a tilled field, not a fruit-tree, not a living being, either human or animal. The once smiling scene was a stretch of fire-blackened desolation. And such things were to be seen not in one region only, but throughout all Germany. Never has that great country known so terrible a state of affairs as she had to face at the end of the Thirty Years' War.