After the loss of Mentz, misfortune still pursued the Spaniards on the Rhine.  Shortly before the capture of that city, the Landgrave of Hesse Cassel had taken Falkenstein and Reifenberg, and the fortress of Koeningstein surrendered to the Hessians.  The Rhinegrave, Otto Louis, one of the king’s generals, defeated nine Spanish squadrons who were on their march for Frankenthal, and made himself master of the most important towns upon the Rhine, from Boppart to Bacharach.  After the capture of the fortress of Braunfels, which was effected by the Count of Wetterau, with the cooeperation of the Swedes, the Spaniards quickly lost every place in Wetterau, while in the Palatine they retained few places besides Frankenthal.  Landau and Kronweisenberg openly declared for the Swedes; Spires offered troops for the king’s service; Mannheim was gained through the prudence of the Duke Bernard of Weimar and the negligence of its governor, who, for this misconduct, was tried before the council of war, at Heidelberg, and beheaded.

The king had protracted the campaign into the depth of winter, and the severity of the season was perhaps one cause of the advantage his soldiers gained over those of the enemy.  But the exhausted troops now stood in need of the repose of winter quarters, which, after the surrender of Mentz, Gustavus assigned to them in its neighborhood.  He himself employed the interval of inactivity in the field, which the season of the year enjoined, in arranging, with his chancellor, the affairs of his cabinet, in treating for a neutrality with some of his enemies, and adjusting some political disputes which had sprung up with a neighboring ally.  He chose the city of Mentz for his winter quarters, and the settlement of these state affairs, and showed a greater partiality for this town than seemed consistent with the interests of the German princes, or the shortness of his visit to the Empire.  Not content with strongly fortifying it, he erected at the opposite angle which the Main forms with the Rhine, a new citadel, which was named Gustavusburg from its founder, but which is better known under the title of Pfaffenraub or Pfaffenzwang.[60]

While Gustavus Adolphus made himself master of the Rhine, and threatened the three neighboring electorates with his victorious arms, his vigilant enemies in Paris and St. Germain’s made use of every artifice to deprive him of the support of France, and, if possible, to involve him in a war with that power.  By his sudden and equivocal march to the Rhine, he had surprised his friends, and furnished his enemies with the means of exciting a distrust of his intentions.  After the conquest of Wuertzburg, and of the greater part of Franconia, the road into Bavaria and Austria lay open to him through Bamberg and the Upper Palatinate; and the expectation was as general, as it was natural, that he would not delay to attack the Emperor and the Duke of Bavaria in the very centre of their power, and, by the reduction of his two principal enemies,