Could he succeed in doing this he would be in a position to dictate terms to the emperor in Vienna.
On the 12th of October he reached Neuberg, on the Danube, and halted there, awaiting the arrival of his siege train from Donauworth.
While making the most vigorous exertions to press on the necessary arrangements for his march against Vienna he received the most urgent messages to return to Saxony.

Not only, as he was told, had Wallenstein penetrated into that province, but he was employing all his influence to detach its elector from the Protestant cause, and there was great fear that the weak prince would yield to the solicitations of Wallenstein and to his own jealousy of the King of Sweden.
No sooner, in fact, had Gustavus crossed the Danube than Wallenstein moved towards Schweinfurt, and by so doing drew to that place the Swedish army under the command of Duke Bernhard.

He then suddenly marched eastward at full speed, capturing Bamberg, Baireuth, and Culmbach, and pushed on to Colberg.
The town was captured, but the Swedish Colonel Dubatel, who was really a Scotchman, by name M'Dougal, a gallant and brilliant officer, threw himself with his dragoons into the castle, which commanded the town, and defended it so resolutely against the assaults of Wallenstein that Duke Bernhard had time to march to within twenty miles of the place.
Wallenstein then raised the siege, marched east to Kronach, and then north to Weida, on the Elster.

Thence he pressed on direct to Leipzig, which he besieged at once; and while the main body of his troops were engaged before the city, others took possession of the surrounding towns and fortresses.
Leipzig held out for only two days, and after its capture Wallenstein marched to Merseburg, where he was joined by the army under Pappenheim.
Thus reinforced he was in a position to capture the whole of Saxony.

The elector, timid and vacillating, was fully conscious of his danger and the solicitations of Wallenstein to break off from his alliance with the King of Sweden and to join the Imperialists were strongly seconded by Marshal Von Arnheim, his most trusted councillor, who was an intimate friend of the Imperialist general.
It was indeed a hard decision which Gustavus was called upon to make.