By the retreat of Torstensohn, the Emperor was relieved from all fears of an irruption on the side of Bohemia. But a new danger soon threatened the Austrian frontier from Suabia and Bavaria. Turenne, who had separated from Conde, and taken the direction of Suabia, had, in the year 1645, been totally defeated by Mercy, near Mergentheim; and the victorious Bavarians, under their brave leader, poured into Hesse. But the Duke of Enghien hastened with considerable succours from Alsace, Koenigsmark from Moravia, and the Hessians from the Rhine, to recruit the defeated army, and the Bavarians were in turn compelled to retire to the extreme limits of Suabia. Here they posted themselves at the village of Allersheim, near Nordlingen, in order to cover the Bavarian frontier. But no obstacle could check the impetuosity of the Duke of Enghien. In person, he led on his troops against the enemy's entrenchments, and a battle took place, which the heroic resistance of the Bavarians rendered most obstinate and bloody; till at last the death of the great Mercy, the skill of Turenne, and the iron firmness of the Hessians, decided the day in favour of the allies. But even this second barbarous sacrifice of life had little effect either on the course of the war, or on the negociations for peace. The French army, exhausted by this bloody engagement, was still farther weakened by the departure of the Hessians, and the Bavarians being reinforced by the Archduke Leopold, Turenne was again obliged hastily to recross the Rhine.
The retreat of the French, enabled the enemy to turn his whole force upon the Swedes in Bohemia. Gustavus Wrangel, no unworthy successor of Banner and Torstensohn, had, in 1646, been appointed Commander-in-chief of the Swedish army, which, besides Koenigsmark's flying corps and the numerous garrisons disposed throughout the empire, amounted to about 8,000 horse, and 15,000 foot. The Archduke, after reinforcing his army, which already amounted to 24,000 men, with twelve Bavarian regiments of cavalry, and eighteen regiments of infantry, moved against Wrangel, in the hope of being able to overwhelm him by his superior force before Koenigsmark could join him, or the French effect a diversion in his favour. Wrangel, however, did not await him, but hastened through Upper Saxony to the Weser, where he took Hoester and Paderborn. From thence he marched into Hesse, in order to join Turenne, and at his camp at Wetzlar, was joined by the flying corps of Koenigsmark. But Turenne, fettered by the instructions of Mazarin, who had seen with jealousy the warlike prowess and increasing power of the Swedes, excused himself on the plea of a pressing necessity to defend the frontier of France on the side of the Netherlands, in consequence of the Flemings having failed to make the promised diversion. But as Wrangel continued to press his just demand, and a longer opposition might have excited distrust on the part of the Swedes, or induce them to conclude a private treaty with Austria, Turenne at last obtained the wished for permission to join the Swedish army.
The junction took place at Giessen, and they now felt themselves strong enough to meet the enemy. The latter had followed the Swedes into Hesse, in order to intercept their commissariat, and to prevent their union with Turenne. In both designs they had been unsuccessful; and the Imperialists now saw themselves cut off from the Maine, and exposed to great scarcity and want from the loss of their magazines. Wrangel took advantage of their weakness, to execute a plan by which he hoped to give a new turn to the war. He, too, had adopted the maxim of his predecessor, to carry the war into the Austrian States. But discouraged by the ill success of Torstensohn's enterprise, he hoped to gain his end with more certainty by another way. He determined to follow the course of the Danube, and to break into the Austrian territories through the midst of Bavaria. A similar design had been formerly conceived by Gustavus Adolphus, which he had been prevented carrying into effect by the approach of Wallenstein's army, and the danger of Saxony. Duke Bernard moving in his footsteps, and more fortunate than Gustavus, had spread his victorious banners between the Iser and the Inn; but the near approach of the enemy, vastly superior in force, obliged him to halt in his victorious career, and lead back his troops. Wrangel now hoped to accomplish the object in which his predecessors had failed, the more so, as the Imperial and Bavarian army was far in his rear upon the Lahn, and could only reach Bavaria by a long march through Franconia and the Upper Palatinate. He moved hastily upon the Danube, defeated a Bavarian corps near Donauwerth, and passed that river, as well as the Lech, unopposed. But by wasting his time in the unsuccessful siege of Augsburg, he gave opportunity to the Imperialists, not only to relieve that city, but also to repulse him as far as Lauingen. No sooner, however, had they turned towards Suabia, with a view to remove the war from Bavaria, than, seizing the opportunity, he repassed the Lech, and guarded the passage of it against the Imperialists themselves. Bavaria now lay open and defenceless before him; the French and Swedes quickly overran it; and the soldiery indemnified themselves for all dangers by frightful outrages, robberies, and extortions. The arrival of the Imperial troops, who at last succeeded in passing the Lech at Thierhaupten, only increased the misery of this country, which friend and foe indiscriminately plundered.
And now, for the first time during the whole course of this war, the courage of Maximilian, which for eight-and-twenty years had stood unshaken amidst fearful dangers, began to waver. Ferdinand II., his school-companion at Ingoldstadt, and the friend of his youth, was no more; and with the death of his friend and benefactor, the strong tie was dissolved which had linked the Elector to the House of Austria. To the father, habit, inclination, and gratitude had attached him; the son was a stranger to his heart, and political interests alone could preserve his fidelity to the latter prince.
Accordingly, the motives which the artifices of France now put in operation, in order to detach him from the Austrian alliance, and to induce him to lay down his arms, were drawn entirely from political considerations. It was not without a selfish object that Mazarin had so far overcome his jealousy of the growing power of the Swedes, as to allow the French to accompany them into Bavaria. His intention was to expose Bavaria to all the horrors of war, in the hope that the persevering fortitude of Maximilian might be subdued by necessity and despair, and the Emperor deprived of his first and last ally. Brandenburg had, under its great sovereign, embraced the neutrality; Saxony had been forced to accede to it; the war with France prevented the Spaniards from taking any part in that of Germany; the peace with Sweden had removed Denmark from the theatre of war; and Poland had been disarmed by a long truce. If they could succeed in detaching the Elector of Bavaria also from the Austrian alliance, the Emperor would be without a friend in Germany and left to the mercy of the allied powers.
Ferdinand III. saw his danger, and left no means untried to avert it. But the Elector of Bavaria was unfortunately led to believe that the Spaniards alone were disinclined to peace, and that nothing, but Spanish influence, had induced the Emperor so long to resist a cessation of hostilities. Maximilian detested the Spaniards, and could never forgive their having opposed his application for the Palatine Electorate. Could it then be supposed that, in order to gratify this hated power, he would see his people sacrificed, his country laid waste, and himself ruined, when, by a cessation of hostilities, he could at once emancipate himself from all these distresses, procure for his people the repose of which they stood so much in need, and perhaps accelerate the arrival of a general peace? All doubts disappeared; and, convinced of the necessity of this step, he thought he should sufficiently discharge his obligations to the Emperor, if he invited him also to share in the benefit of the truce.