As soon as the plan of operations had been concerted at Halle, between the King of Sweden and the Elector of Saxony; as soon as the alliance had been concluded with the neighboring princes of Weimar and Anhalt, and preparations made for the recovery of the bishopric of Magdeburg, the king began his march into the empire.  He had here no despicable foe to contend with.  Within the empire, the Emperor was still powerful; throughout Franconia, Swabia, and the Palatinate, imperial garrisons were posted, with whom the possession of every place of importance must be disputed sword in hand.  On the Rhine he was opposed by the Spaniards, who had overrun the territory of the banished Elector Palatine, seized all its strong places, and would everywhere dispute with him the passage over the river.  In his rear was Tilly, who was fast recruiting his force, and would soon be joined by the auxiliaries from Lorraine.  Every Papist presented an inveterate foe, while his connection with France did not leave him at liberty to act with freedom against the Roman Catholics.  Gustavus had foreseen all these obstacles, and at the same time the means by which they were to be overcome.  The strength of the Imperialists was broken and divided among different garrisons, while he would bring against them one by one his whole united force.  If he was to be opposed by the fanaticism of the Roman Catholics, and the awe in which the lesser states regarded the Emperor’s power, he might depend on the active support of the Protestants, and their hatred to Austrian oppression.  The ravages of the Imperialists and Spanish troops also powerfully aided him in these quarters where the ill-treated husbandman and citizen alike sighed for a deliverer, and where the mere change of yoke seemed to promise a relief.  Emissaries were dispatched to gain over to the Swedish side the principal free cities, particularly Nuremberg and Frankfort.  The first that lay in the king’s march, and which he could not leave unoccupied in his rear, was Erfurt.  Here the Protestant party among the citizens opened to him, without a blow, the gates of the town and the citadel.  From the inhabitants of this, as of every important place which afterward submitted, he exacted an oath of allegiance, while he secured its possession by a sufficient garrison.  To his ally, Duke William of Weimar, he intrusted the command of an army to be raised in Thuringia.  He also left his queen in Erfurt, and promised to increase its privileges.  The Swedish army now crossed the Thuringian forest in two columns, by Gotha and Arnstadt, and having delivered, in its march, the county of Henneberg from the Imperialists, formed a junction on the third day near Koenigshofen, on the frontiers of Franconia.