At Erfurt Gustavus was joined by Duke Bernhard of Saxe-Weimar with his force, which raised his army to a strength of 20,000. The news of his approach had again revived the courage of the Elector of Saxony, who had occupied the only towns where the Elbe could be crossed, Dresden, Torgau, and Wittenberg--he himself, with his main army of 15,000 men, lying at Torgau. From him Gustavus learned that the Imperial army was divided into three chief corps--that of Wallenstein 12,000 strong, that of Pappenheim 10,000, those of Gallas and Holk united 16,000, making a total of 38,000 men. So great was the speed with which Gustavus had marched to Erfurt that Wallenstein had received no notice of his approach; and believing that for some time to come he should meet with no serious opposition, he had on the very day after the Swedes reached Erfurt despatched Gallas with 12,000 men into Bohemia. A division of his troops was at the same time threatening Naumburg, whose possession would enable him to block the only easy road with which Gustavus could enter the country held by him. But Gustavus at Erfurt learned that Naumburg had not yet fallen, and marching with great rapidity reached the neighbourhood of that town before the Imperialists were aware that he had quitted Erfurt, and cutting up a small detachment of the enemy who lay in his way, entered the town and at once began to intrench it. Wallenstein first learned from the fugitives of the beaten detachment that Gustavus had arrived at Naumburg, but as his own position lay almost centrally between Naumburg and Torgau, so long as he could prevent the Swedes and Saxons from uniting, he felt safe; for although together they would outnumber him, he was superior in strength to either if alone. The Imperialist general believed that Gustavus intended to pass the winter at Naumburg, and he had therefore no fear of an immediate attack. In order to extend the area from which he could draw his supplies Wallenstein despatched Pappenheim to secure the fortress of Halle; for although that town had been captured the fortress held out, and barred the main road to the north. From Halle Pappenheim was to proceed to the relief of Cologne, which was menaced by the enemy.