During the early phases of the Swedish invasion, Tilly had remained inactive in western Germany, awaiting the return of the Imperial forces detached to Italy.
By late 1630 however, his reinforced armies moved east. By mid-January, they had arrived before Frankfort-an-der-Oder to reinforce the depleted Imperial forces.
With the Oder clear all the way to Frankfort-an-der-Oder and with the remaining Imperial garrisons of Pomerania fled in January, 1631 Gustavus struck west into Mecklenburg. In contrast to his prior feint, he was able to conquer the place completely.
Within Pomerania and Mecklenburg, the Imperials held only the fortresses of Demmin, Greifswald and Kolberg, all of which fell in March, 1631.
The arrival of the Swedish “liberator” presented the Protestant princes with a conundrum. The Leipzig Colloquy represented a fruitless attempt to create a “third force” within the Empire which would provide a counterbalance to Imperial and foreign power.
At the urging of Georg-Wilhelm of Brandenburg, Johann-Georg of Saxony convened a meeting of the Protestant princes at Leipzig. The Leipzig Colloquy opened in Februrary, 1631. Both Protestant Electors, Johann-Georg himself and Georg-Wilhelm, attended, as did most of the lesser Saxon princes, the administrators of the Protestant bishoprics and Imperial Free Cities.
The Protestant rulers determined to ally amongst themselves and raise forces to defend themselves. They also hoped this armed alliance, the Leipziger Bund, would enable them to bargain with the Emperor.
Thus the assembled sent a manifesto to the Emperor demanding that he rescind the Edict of Restitution. If he did so, they promised to join the Imperial forces in a campaign against the Swedes.
Johann-Georg himself raised an army of 40,000 men; the other princes smaller forces. Johann-Georg placed his forces under the command of Hans-Georg von Arnim, an able general who had formerly been second-in-command to Wallenstein.
The Emperor refused to countenance any change to the Edict. His Jesuit confessor Lamormain had convinced him that it was better to lose all his kingdoms in this life than to lose his hope of eternal salvation.
Tilly had moved his forces east from the Weser, joining in January, 1631 the remnants of the Imperial forces in their refuge at Frankfort-an-der-Oder. However, fearing that Gustavus would strike west toward the Elbe and Magdeburg, he returned east across Brandenburg and joined his forces with those of von Pappenheim. The siege of Magdeburg had begun.
Gustavus, having secured the Oder valley and destroyed the Imperial forces at Frankfort-an-der-Oder turned west towards Magdeburg. However, the equivocal attitude of the Electors of Brandenburg and Saxony limited his ability to strike toward the city. If they made their peace with the Emperor, he would be cut off from the sea and Sweden.
Gustavus moved forward through Brandenburg. The Elector was induced by threats to allow Gustavus to garrison Küstrin and Spandau until Magdeburg should be relieved.
Gustavus had moved too slowly. Under attack by the entire Imperial army, the city was unable to resist. On 20 May, 1631, Magdeburg fell.
During the ensuing sack the city was engulfed in flame. All of Magdeburg was destroyed and 20,000 of her inhabitants with her. Tilly, who had thought to sustain his army on the supplies within the city, was left master of a wasteland.
News of the Sack of Magdeburg resounded throughout the Empire. The destruction of the foremost city of Lutheranism alarmed the Protestant princes. In accordance with their agreement at Leipzig, the Protestant rulers of Swabia and Franconia began to levy troops.
Gustavus was also spurred on. After the sack, he punctiliously handed back to the Elector of Brandenburg Küstrin and Spandau, which he had garrisoned only for the duration of the siege of the doomed city. He then marched his army to the walls of Berlin. With Gustavus’s army surrounding his capital, the merits of a Swedish alliance became clear to Georg-Wilhelm. The treaty was signed on 19 June, 1631, and Brandenburg was obliged to pay monthly 30,000 Reichsthaler into the chronically empty Swedish warchest.
Tilly, in contrast, seemed almost stunned by the enormity of the destruction. He lingered immobile in the smoking ruins of Magdeburg until the end of June.
Then he marched west to Swabia and Franconia to join with the Imperial troops newly released from service in Italy. There the Protestant states had raised armies as they had agreed at the Leipzig Colloquy. Bernard of Saxe-Weimar’s army was occupying the Bishopric of Fulda, and Gustavus’s ally Wilhelm of Hesse-Cassel was also in arms.
The western expedition succeeded in cowing the Protestants into submission, except for Hesse-Cassel, who was driven to refuge in his fortresses.
The Swedes, encouraged by Tilly’s absence, had overrun Wallenstein’s Duchy of Mecklenburg and re-installed the ousted dukes. They had also moved forward to the Elbe, taking Werben threatening the Imperial forces. To deal with this threat to his rear in August, 1631, Tilly turned his forces eastward.
Johann-Georg, Elector of Saxony, found himself in a difficult position. He, like the rest of the German princes, had no desire for a Swedish alliance. The Emperor had demanded Johann-Georg disband his army and Tilly had demanded that the starving Imperial armies be quartered on the rich Saxon lands. Johann-Georg had to choose King or Emperor.
Tilly moved his army forward into Southern Saxony, occupying Merseburg and Halle.
Johann-Georg made his choice: On 30 August 1631 he promised alliance to Gustavus. The Swedish and Saxon armies, now joined, pursued Tilly to Leipzig, which had surrendered to the Imperials.
Tilly, outnumbered by the allied force, was reluctant to give battle. He thought to break the Suedo-Saxon forces by a long siege of Leipzig.
Von Pappenheim had been sent on a sortie against the allied forces. When the came in contact with them he claimed to be unable to break contact and demanded that Tilly bring forward the remainder of the Imperial force to support him. Tilly, reluctant, agreed.
The armies drew up on a plain near the village of Breitenfeld, a few miles outside the walls of Leipzig. At first, von Pappenheim’s reckless gamble seemed about to pay off: at the first charge of his cavalry, the raw Saxon levies broke and fled the field. However, Gustavus nimbly turned his line to prevent the ponderous battalia of the Imperial forces flanking him. A daring Swedish cavalry raid captured all of the Imperial artillery, which was turned against its owners. Artillery pounded the Imperial phalanxes until night fell.
The Imperial armies were utterly destroyed. They lost two-thirds of their men and all their artillery and supplies. The remnants fled: von Pappenheim took refuge in Westphalia behind the line of the Weser while Tilly fell back on the Upper Palatinate.
In a stroke, Gustavus had made himself master of Germany.
Before the Battle of Breitenfeld, Gustavus had promised his troops that if they were victorious, the plunder of the rich Rhenish bishoprics of “Priests’ Alley” would be theirs. He moved his army west to redeem the pledge.
His Saxon allies, and a small Swedish contingent, were sent southeast through Lusatia to invade the Habsburg lands of Bohemia and Moravia. Wallenstein had retired there after his dismissal and it was thought that he might join the Saxons, commanded by his former lieutenant von Arnim, in an attack on the Emperor.
Gustavus sent his forces hurrying westward through Franconia, Thuringia and down the valleys of the Main and Rhine. In October, he took the rich Catholic bishopric of Würzberg, in November he was in the Archbishopric of the Elector of Mainz, by December, his troops had entered the Lower Palatinate.
A smaller northern force under Banér and Tott pushed across Mecklenburg to the Weser, and put violated Magdeburg once more in Lutheran hands.
At the same time as he was occupying the lands of the Catholic prelates, Gustavus was also attracting Protestant rulers to his cause: the Princes of Anhalt, the Weimar Dukes, the Duke of Württemburg, the Duke of Brunswick-Lüneberg and the Imperial free city of Nürnberg all joined his banner.
Tilly’s decimated forces were able to make no more than spoiling attacks in the south. Pushes into occupied Würzberg and against Nürnberg were easily repulsed. Tilly withdrew into Bavaria to join Maximilian’s armies for a last-ditch defense of Bavaria.
Gustavus took up winter quarters at Mainz. There he built the vast camp Gustavusburg as home for his conquering army.
Meanwhile, the Saxon army under Arnim, together with 12,00 Swedish troops were moving east. In October they moved down through Lusatia, into Moravia and Bohemia. Nowhere did they meet effective resistance. In November, 1631, Prague fell without a blow struck.