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.