Crouched on a promontory above the Rhine, the great fortress of Breisach was the key to control of Voröstereich, the Habsburg lands scattered along the Rhine.
Despite his ultimate victory, Bernhard of Saxe-Weimar’s army had been weakened by the battles at Rheinfelden. It was not until May, 1638, after he had received French reinforcements under de Guébriant, that Saxe-Weimar was in a position to exploit his victory by moving against Breisach.
In the meantime the Imperials, alive to their peril, had sent the Bavarian general Götz into the Black Forest to reinforce Breisach.
Saxe-Weimar besieged the fortress in June, 1638 but found himself too weak and withdrew.
In August, having received further French reinforcement under the young Turenne, Bernhard returned to his task.
As Bernhard moved forward, the Bavarian Götz and the Imperial general Savello were dispatched with a convoy of reinforcements for Breisach.
As they moved toward the fortress Bernhard was waiting for them. On on 8 August, 1638 Saxe-Weimar’s numerically inferior forces defeated first Savello and then Götz in detail as their armies debouched from the defile at Wittenweier.
Having shattered the reinforcing Imperial forces at Wittenweier, on 18 August, 1638 Bernhard encircled impregnable Breisach and prepared to starve its garrison out.
The Emperor desperately attempted to relieve the siege. Bernhard was easily able to turn back these spasmodic and ill-coördinated attacks. Through the autumn of 1638 the fortress starved.
Finally on 17 December, 1638 Breisach’s Governor von Rheinach capitulated. He was granted honorable terms.
The lenient terms were nearly revoked when Bernhard discovered that some of his men held prisoner in the fortress had been driven by shortness of provision to cannibalize their dead comrades.
Axel Oxenstierna had fondly hoped that he could remain independent of French policy and reach a separate peace with the Emperor. He had also fondly hoped that the German war could be fought by German troops and without further effusion of Swedish blood.
With the Swedes barely clinging to the shores of Pomerania, Oxenstierna realized neither of these hopes would be realized.
Firstly, the Swedes stood in need of French money. In March, 1638 the prolonged Franco-Swedish negotiations at Hamburg finally bore fruit with the signing of the Treaty of Hamburg.
Each side agreed that during the three-year term of the treaty no separate peace with the Emperor would be made by either.
The French were to pay the Swedes an annual subsidy of one million livres. In return, the Swedes were to carry the war into the Habsburg dominions to the East while the French continued their fight in the Rhineland.
Oxenstierna also sent Banér 14,000 fresh recruits from the homeland.
With French gold in his purse and honest Swedish bönder at his back, by July, 1638 Banér had easily cleared Mecklenburg and Pomerania of Imperial troops. He then drove the Imperial forces under Gallas back into Silesia and Bohemia.
The only blot on 1638’s record of anti-Imperial advance was provided by the house of the Elector Palatine. Karl-Ludwig, eldest son of the late Winter King, conceived a quixotic plan to recover his patrimony.
Subsidized by the English, he raised an army to attack along the River Ems. The result was in the proud traditions of ineffective Stuart foreign policy and of Karl-Ludwig’s feckless house. In October, 1638 the army was met at Hochfeld and annihilated.
Despite the fall of Hesse-Cassel to the Imperial forces, Amalia-Elisabeth still had a powerful army encamped in her lands in East Friesland. In 1638, with this in mind, the Emperor granted her amnesty and a return of the lands of her late husband.
In August, 1638, the Emperor’s emissaries entered into the Treaty of Mainz which would have given Calvinists such as Amalia-Elisabeth rights under the Imperial constitution equal to those of Lutherans. However, while the Emperor was willing to treat with individual Calvinists, a blanket indemnity was unacceptable to him and he refused to ratify the agreement.
After his conquest of Breisach, Saxe-Weimar and his French paymasters began to dispute control of the place. Bernhard asserted that the Landgraviate of Alsace the French had promised him encompassed the fortress. The French for their part claimed that Breisach was part of the Breisgau and not of Alsace at all, and that, in any event, the Landgraviate did not carry with it control of the Alsatian fortresses, which was to remain with France.
Although the French and Swedes had planned that their forces were to join for the 1639 campaign, in January, 1639 the discontented Saxe-Weimar withdrew his army to the Franche-Comté where he sulked like Achilles in his tent.
It was not until July, 1639 that Saxe-Weimar moved his forces, perhaps to battle or perhaps to escape the plague which had broken out in Pontarlier. If it was the latter, he fled in vain: on 11 July, 1639 Bernhard of Saxe-Weimar died of the pest.
Although their commander was dead, Saxe-Weimar’s army remained a potent force. Erlach, Saxe-Weimar’s politically-astute second-in-command, began to negotiate with potential paymasters. The Swedes were interested, as was Karl-Ludwig of the Palatinate. Even the Emperor was interested in buying out his enemy.
In the end, Erlach and the other commanders determined to serve the French. On 9 October, 1639 Erlach and Louis XIII entered into an agreement under which the troops were to continue as a separate unit in the service of France.
Matters thus settled, the French and their new auxiliaries ended the inaction that had followed from the Breisach disputes. Their combined forces under Longueville marched down the Rhine, taking Germersheim, Bingen and Kreuznach, and crossed the river at Bacharach and Overwesel.
While the French arms in the south remained paralyzed by the Breisach dispute, Banér pursued an active campaign in the north.
First, in January, 1639 he swung west through the Brunswick duchies, perhaps in the hope that Saxe-Weimar would join him or perhaps to overawe the unreliable Georg of Brunswick-Lüneberg. When Bernhard did not appear, he turned east against the Saxons and their Imperial allies.
The Swedes drove the Saxons back to Dresden. At Chemnitz, Saxony they met the Imperials under Ferdinand III’s brother the Archduke Leopold-Wilhelm. On 14 April, 1639 they inflicted a crushing defeat on the Archduke, opening the road into Silesia and Bohemia.
Banér continued east, taking Pirna and breaking another Imperial army near Brandeis. By May, 1639, he was before the walls of Prague. However, he was unable to sustain the siege and eventually withdrew to the Elbe.
With the unpleasantness over Breisach ended, the French and Swedes determined to campaign together in central Germany, as they had planned to in 1639.
The successes of 1639 had also brought the firmer accession of German allies. The foreigners would be joined by the armies of Hesse-Cassel under Melander and of Brunswick-Lüneberg under von Klitzing.
However, after this powerful force assembled at Erfurt, the results were less than impressive. The Imperial forces under Piccolomini wisely refused battle. Banér did not press the issue, and drifted aimlessly through central Germany toward the Weser. In the end, nothing was accomplished.