He had attacked, and on the 16th April, carried Landsberg, when he was apprised of the danger of Magdeburg. He resolved immediately to march to the relief of that town; and he moved with all his cavalry, and ten regiments of infantry towards the Spree. But the position which he held in Germany, made it necessary that he should not move forward without securing his rear. In traversing a country where he was surrounded by suspicious friends and dangerous enemies, and where a single premature movement might cut off his communication with his own kingdom, the utmost vigilance and caution were necessary. The Elector of Brandenburg had already opened the fortress of Custrin to the flying Imperialists, and closed the gates against their pursuers. If now Gustavus should fail in his attack upon Tilly, the Elector might again open his fortresses to the Imperialists, and the king, with an enemy both in front and rear, would be irrecoverably lost. In order to prevent this contingency, he demanded that the Elector should allow him to hold the fortresses of Custrin and Spandau, till the siege of Magdeburg should be raised.

Nothing could be more reasonable than this demand. The services which Gustavus had lately rendered the Elector, by expelling the Imperialists from Brandenburg, claimed his gratitude, while the past conduct of the Swedes in Germany entitled them to confidence. But by the surrender of his fortresses, the Elector would in some measure make the King of Sweden master of his country; besides that, by such a step, he must at once break with the Emperor, and expose his States to his future vengeance. The Elector's struggle with himself was long and violent, but pusillanimity and self-interest for awhile prevailed. Unmoved by the fate of Magdeburg, cold in the cause of religion and the liberties of Germany, he saw nothing but his own danger; and this anxiety was greatly stimulated by his minister Von Schwartzenburgh, who was secretly in the pay of Austria. In the mean time, the Swedish troops approached Berlin, and the king took up his residence with the Elector. When he witnessed the timorous hesitation of that prince, he could not restrain his indignation: "My road is to Magdeburg," said he; "not for my own advantage, but for that of the Protestant religion. If no one will stand by me, I shall immediately retreat, conclude a peace with the Emperor, and return to Stockholm. I am convinced that Ferdinand will readily grant me whatever conditions I may require. But if Magdeburg is once lost, and the Emperor relieved from all fear of me, then it is for you to look to yourselves and the consequences." This timely threat, and perhaps, too, the aspect of the Swedish army, which was strong enough to obtain by force what was refused to entreaty, brought at last the Elector to his senses, and Spandau was delivered into the hands of the Swedes.

The king had now two routes to Magdeburg; one westward led through an exhausted country, and filled with the enemy's troops, who might dispute with him the passage of the Elbe; the other more to the southward, by Dessau and Wittenberg, where bridges were to be found for crossing the Elbe, and where supplies could easily be drawn from Saxony. But he could not avail himself of the latter without the consent of the Elector, whom Gustavus had good reason to distrust. Before setting out on his march, therefore, he demanded from that prince a free passage and liberty for purchasing provisions for his troops. His application was refused, and no remonstrances could prevail on the Elector to abandon his system of neutrality. While the point was still in dispute, the news of the dreadful fate of Magdeburg arrived.

Tilly announced its fall to the Protestant princes in the tone of a conqueror, and lost no time in making the most of the general consternation. The influence of the Emperor, which had sensibly declined during the rapid progress of Gustavus, after this decisive blow rose higher than ever; and the change was speedily visible in the imperious tone he adopted towards the Protestant states. The decrees of the Confederation of Leipzig were annulled by a proclamation, the Convention itself suppressed by an imperial decree, and all the refractory states threatened with the fate of Magdeburg. As the executor of this imperial mandate, Tilly immediately ordered troops to march against the Bishop of Bremen, who was a member of the Confederacy, and had himself enlisted soldiers. The terrified bishop immediately gave up his forces to Tilly, and signed the revocation of the acts of the Confederation. An imperial army, which had lately returned from Italy, under the command of Count Furstenberg, acted in the same manner towards the Administrator of Wirtemberg. The duke was compelled to submit to the Edict of Restitution, and all the decrees of the Emperor, and even to pay a monthly subsidy of 100,000 dollars, for the maintenance of the imperial troops. Similar burdens were inflicted upon Ulm and Nuremberg, and the entire circles of Franconia and Swabia. The hand of the Emperor was stretched in terror over all Germany. The sudden preponderance, more in appearance, perhaps, than in reality, which he had obtained by this blow, carried him beyond the bounds even of the moderation which he had hitherto observed, and misled him into hasty and violent measures, which at last turned the wavering resolution of the German princes in favour of Gustavus Adolphus. Injurious as the immediate consequences of the fall of Magdeburg were to the Protestant cause, its remoter effects were most advantageous. The past surprise made way for active resentment, despair inspired courage, and the German freedom rose, like a phoenix, from the ashes of Magdeburg.

Among the princes of the Leipzig Confederation, the Elector of Saxony and the Landgrave of Hesse were the most powerful; and, until they were disarmed, the universal authority of the Emperor was unconfirmed. Against the Landgrave, therefore, Tilly first directed his attack, and marched straight from Magdeburg into Thuringia. During this march, the territories of Saxe Ernest and Schwartzburg were laid waste, and Frankenhausen plundered before the very eyes of Tilly, and laid in ashes with impunity. The unfortunate peasant paid dear for his master's attachment to the interests of Sweden. Erfurt, the key of Saxony and Franconia, was threatened with a siege, but redeemed itself by a voluntary contribution of money and provisions. From thence, Tilly despatched his emissaries to the Landgrave, demanding of him the immediate disbanding of his army, a renunciation of the league of Leipzig, the reception of imperial garrisons into his territories and fortresses, with the necessary contributions, and the declaration of friendship or hostility. Such was the treatment which a prince of the Empire was compelled to submit to from a servant of the Emperor. But these extravagant demands acquired a formidable weight from the power which supported them; and the dreadful fate of Magdeburg, still fresh in the memory of the Landgrave, tended still farther to enforce them. Admirable, therefore, was the intrepidity of the Landgrave's answer: "To admit foreign troops into his capital and fortresses, the Landgrave is not disposed; his troops he requires for his own purposes; as for an attack, he can defend himself. If General Tilly wants money or provisions, let him go to Munich, where there is plenty of both." The irruption of two bodies of imperial troops into Hesse Cassel was the immediate result of this spirited reply, but the Landgrave gave them so warm a reception that they could effect nothing; and just as Tilly was preparing to follow with his whole army, to punish the unfortunate country for the firmness of its sovereign, the movements of the King of Sweden recalled him to another quarter.

Gustavus Adolphus had learned the fall of Magdeburg with deep regret; and the demand now made by the Elector, George William, in terms of their agreement, for the restoration of Spandau, greatly increased this feeling. The loss of Magdeburg had rather augmented than lessened the reasons which made the possession of this fortress so desirable; and the nearer became the necessity of a decisive battle between himself and Tilly, the more unwilling he felt to abandon the only place which, in the event of a defeat, could ensure him a refuge. After a vain endeavour, by entreaties and representations, to bring over the Elector to his views, whose coldness and lukewarmness daily increased, he gave orders to his general to evacuate Spandau, but at the same time declared to the Elector that he would henceforth regard him as an enemy.

To give weight to this declaration, he appeared with his whole force before Berlin. "I will not be worse treated than the imperial generals," was his reply to the ambassadors whom the bewildered Elector despatched to his camp. "Your master has received them into his territories, furnished them with all necessary supplies, ceded to them every place which they required, and yet, by all these concessions, he could not prevail upon them to treat his subjects with common humanity. All that I require of him is security, a moderate sum of money, and provisions for my troops; in return, I promise to protect his country, and to keep the war at a distance from him. On these points, however, I must insist; and my brother, the Elector, must instantly determine to have me as a friend, or to see his capital plundered." This decisive tone produced a due impression; and the cannon pointed against the town put an end to the doubts of George William. In a few days, a treaty was signed, by which the Elector engaged to furnish a monthly subsidy of 30,000 dollars, to leave Spandau in the king's hands, and to open Custrin at all times to the Swedish troops. This now open alliance of the Elector of Brandenburg with the Swedes, excited no less displeasure at Vienna, than did formerly the similar procedure of the Duke of Pomerania; but the changed fortune which now attended his arms, obliged the Emperor to confine his resentment to words.

The king's satisfaction, on this favourable event, was increased by the agreeable intelligence that Griefswald, the only fortress which the Imperialists still held in Pomerania, had surrendered, and that the whole country was now free of the enemy. He appeared once more in this duchy, and was gratified at the sight of the general joy which he had caused to the people. A year had elapsed since Gustavus first entered Germany, and this event was now celebrated by all Pomerania as a national festival. Shortly before, the Czar of Moscow had sent ambassadors to congratulate him, to renew his alliance, and even to offer him troops. He had great reason to rejoice at the friendly disposition of Russia, as it was indispensable to his interests that Sweden itself should remain undisturbed by any dangerous neighbour during the war in which he himself was engaged. Soon after, his queen, Maria Eleonora, landed in Pomerania, with a reinforcement of 8000 Swedes; and the arrival of 6000 English, under the Marquis of Hamilton, requires more particular notice because this is all that history mentions of the English during the Thirty Years' War.

During Tilly's expedition into Thuringia, Pappenheim commanded in Magdeburg; but was unable to prevent the Swedes from crossing the Elbe at various points, routing some imperial detachments, and seizing several posts. He himself, alarmed at the approach of the King of Sweden, anxiously recalled Tilly, and prevailed upon him to return by rapid marches to Magdeburg. Tilly encamped on this side of the river at Wolmerstadt; Gustavus on the same side, near Werben, not far from the confluence of the Havel and the Elbe. His very arrival portended no good to Tilly. The Swedes routed three of his regiments, which were posted in villages at some distance from the main body, carried off half their baggage, and burned the remainder. Tilly in vain advanced within cannon shot of the king's camp, and offered him battle. Gustavus, weaker by one-half than his adversary, prudently declined it; and his position was too strong for an attack. Nothing more ensued but a distant cannonade, and a few skirmishes, in which the Swedes had invariably the advantage. In his retreat to Wolmerstadt, Tilly's army was weakened by numerous desertions. Fortune seemed to have forsaken him since the carnage of Magdeburg.

The King of Sweden, on the contrary, was followed by uninterrupted success. While he himself was encamped in Werben, the whole of Mecklenburg, with the exception of a few towns, was conquered by his General Tott and the Duke Adolphus Frederick; and he enjoyed the satisfaction of reinstating both dukes in their dominions. He proceeded in person to Gustrow, where the reinstatement was solemnly to take place, to give additional dignity to the ceremony by his presence. The two dukes, with their deliverer between them, and attended by a splendid train of princes, made a public entry into the city, which the joy of their subjects converted into an affecting solemnity. Soon after his return to Werben, the Landgrave of Hesse Cassel appeared in his camp, to conclude an offensive and defensive alliance; the first sovereign prince in Germany, who voluntarily and openly declared against the Emperor, though not wholly uninfluenced by strong motives. The Landgrave bound himself to act against the king's enemies as his own, to open to him his towns and territory, and to furnish his army with provisions and necessaries. The king, on the other hand, declared himself his ally and protector; and engaged to conclude no peace with the Emperor without first obtaining for the Landgrave a full redress of grievances. Both parties honourably performed their agreement. Hesse Cassel adhered to the Swedish alliance during the whole of this tedious war; and at the peace of Westphalia had no reason to regret the friendship of Sweden.