To leave Sweden at full liberty to act against Austria, France commenced her operations by liberating it from all fear of a Polish war. By means of the Count d'Avaux, its minister, an agreement was concluded between the two powers at Stummsdorf in Prussia, by which the truce was prolonged for twenty-six years, though not without a great sacrifice on the part of the Swedes, who ceded by a single stroke of the pen almost the whole of Polish Prussia, the dear-bought conquest of Gustavus Adolphus. The treaty of Beerwald was, with certain modifications, which circumstances rendered necessary, renewed at different times at Compiegne, and afterwards at Wismar and Hamburg. France had already come to a rupture with Spain, in May, 1635, and the vigorous attack which it made upon that power, deprived the Emperor of his most valuable auxiliaries from the Netherlands. By supporting the Landgrave William of Cassel, and Duke Bernard of Weimar, the Swedes were enabled to act with more vigour upon the Elbe and the Danube, and a diversion upon the Rhine compelled the Emperor to divide his force.
The war was now prosecuted with increasing activity. By the treaty of Prague, the Emperor had lessened the number of his adversaries within the Empire; though, at the same time, the zeal and activity of his foreign enemies had been augmented by it. In Germany, his influence was almost unlimited, for, with the exception of a few states, he had rendered himself absolute master of the German body and its resources, and was again enabled to act in the character of emperor and sovereign. The first fruit of his power was the elevation of his son, Ferdinand III., to the dignity of King of the Romans, to which he was elected by a decided majority of votes, notwithstanding the opposition of Treves, and of the heirs of the Elector Palatine. But, on the other hand, he had exasperated the Swedes to desperation, had armed the power of France against him, and drawn its troops into the heart of the kingdom. France and Sweden, with their German allies, formed, from this moment, one firm and compactly united power; the Emperor, with the German states which adhered to him, were equally firm and united. The Swedes, who no longer fought for Germany, but for their own lives, showed no more indulgence; relieved from the necessity of consulting their German allies, or accounting to them for the plans which they adopted, they acted with more precipitation, rapidity, and boldness. Battles, though less decisive, became more obstinate and bloody; greater achievements, both in bravery and military skill, were performed; but they were but insulated efforts; and being neither dictated by any consistent plan, nor improved by any commanding spirit, had comparatively little influence upon the course of the war.
Saxony had bound herself, by the treaty of Prague, to expel the Swedes from Germany. From this moment, the banners of the Saxons and Imperialists were united: the former confederates were converted into implacable enemies. The archbishopric of Magdeburg which, by the treaty, was ceded to the prince of Saxony, was still held by the Swedes, and every attempt to acquire it by negociation had proved ineffectual. Hostilities commenced, by the Elector of Saxony recalling all his subjects from the army of Banner, which was encamped upon the Elbe. The officers, long irritated by the accumulation of their arrears, obeyed the summons, and evacuated one quarter after another. As the Saxons, at the same time, made a movement towards Mecklenburg, to take Doemitz, and to drive the Swedes from Pomerania and the Baltic, Banner suddenly marched thither, relieved Doemitz, and totally defeated the Saxon General Baudissin, with 7000 men, of whom 1000 were slain, and about the same number taken prisoners. Reinforced by the troops and artillery, which had hitherto been employed in Polish Prussia, but which the treaty of Stummsdorf rendered unnecessary, this brave and impetuous general made, the following year (1636), a sudden inroad into the Electorate of Saxony, where he gratified his inveterate hatred of the Saxons by the most destructive ravages. Irritated by the memory of old grievances which, during their common campaigns, he and the Swedes had suffered from the haughtiness of the Saxons, and now exasperated to the utmost by the late defection of the Elector, they wreaked upon the unfortunate inhabitants all their rancour. Against Austria and Bavaria, the Swedish soldier had fought from a sense, as it were, of duty; but against the Saxons, they contended with all the energy of private animosity and personal revenge, detesting them as deserters and traitors; for the hatred of former friends is of all the most fierce and irreconcileable. The powerful diversion made by the Duke of Weimar, and the Landgrave of Hesse, upon the Rhine and in Westphalia, prevented the Emperor from affording the necessary assistance to Saxony, and left the whole Electorate exposed to the destructive ravages of Banner's army.
At length, the Elector, having formed a junction with the Imperial General Hatzfeld, advanced against Magdeburg, which Banner in vain hastened to relieve. The united army of the Imperialists and the Saxons now spread itself over Brandenburg, wrested several places from the Swedes, and almost drove them to the Baltic. But, contrary to all expectation, Banner, who had been given up as lost, attacked the allies, on the 24th of September, 1636, at Wittstock, where a bloody battle took place. The onset was terrific; and the whole force of the enemy was directed against the right wing of the Swedes, which was led by Banner in person. The contest was long maintained with equal animosity and obstinacy on both sides. There was not a squadron among the Swedes, which did not return ten times to the charge, to be as often repulsed; when at last, Banner was obliged to retire before the superior numbers of the enemy. His left wing sustained the combat until night, and the second line of the Swedes, which had not as yet been engaged, was prepared to renew it the next morning. But the Elector did not wait for a second attack. His army was exhausted by the efforts of the preceding day; and, as the drivers had fled with the horses, his artillery was unserviceable. He accordingly retreated in the night, with Count Hatzfeld, and relinquished the ground to the Swedes. About 5000 of the allies fell upon the field, exclusive of those who were killed in the pursuit, or who fell into the hands of the exasperated peasantry. One hundred and fifty standards and colours, twenty-three pieces of cannon, the whole baggage and silver plate of the Elector, were captured, and more than 2000 men taken prisoners. This brilliant victory, achieved over an enemy far superior in numbers, and in a very advantageous position, restored the Swedes at once to their former reputation; their enemies were discouraged, and their friends inspired with new hopes. Banner instantly followed up this decisive success, and hastily crossing the Elbe, drove the Imperialists before him, through Thuringia and Hesse, into Westphalia. He then returned, and took up his winter quarters in Saxony.
But, without the material aid furnished by the diversion upon the Rhine, and the activity there of Duke Bernard and the French, these important successes would have been unattainable. Duke Bernard, after the defeat of Nordlingen, reorganized his broken army at Wetterau; but, abandoned by the confederates of the League of Heilbronn, which had been dissolved by the peace of Prague, and receiving little support from the Swedes, he found himself unable to maintain an army, or to perform any enterprise of importance. The defeat at Nordlingen had terminated all his hopes on the Duchy of Franconia, while the weakness of the Swedes, destroyed the chance of retrieving his fortunes through their assistance. Tired, too, of the constraint imposed upon him by the imperious chancellor, he turned his attention to France, who could easily supply him with money, the only aid which he required, and France readily acceded to his proposals. Richelieu desired nothing so much as to diminish the influence of the Swedes in the German war, and to obtain the direction of it for himself. To secure this end, nothing appeared more effectual than to detach from the Swedes their bravest general, to win him to the interests of France, and to secure for the execution of its projects the services of his arm. From a prince like Bernard, who could not maintain himself without foreign support, France had nothing to fear, since no success, however brilliant, could render him independent of that crown. Bernard himself came into France, and in October, 1635, concluded a treaty at St. Germaine en Laye, not as a Swedish general, but in his own name, by which it was stipulated that he should receive for himself a yearly pension of one million five hundred thousand livres, and four millions for the support of his army, which he was to command under the orders of the French king. To inflame his zeal, and to accelerate the conquest of Alsace, France did not hesitate, by a secret article, to promise him that province for his services; a promise which Richelieu had little intention of performing, and which the duke also estimated at its real worth. But Bernard confided in his good fortune, and in his arms, and met artifice with dissimulation. If he could once succeed in wresting Alsace from the enemy, he did not despair of being able, in case of need, to maintain it also against a friend. He now raised an army at the expense of France, which he commanded nominally under the orders of that power, but in reality without any limitation whatever, and without having wholly abandoned his engagements with Sweden. He began his operations upon the Rhine, where another French army, under Cardinal Lavalette, had already, in 1635, commenced hostilities against the Emperor.