After the battle of Freiburg in early August, the French under the Duc d'Enghien refrained from attacking the city and marched north to besiege the imperial-held Philippsburg instead.

With Philippsburg and the subsequent capture of Mainz, the French took control of the northern Rhine valley, enabling them to launch offensives into the interior of Germany and against Bavaria, the emperor's most important ally.

Afterwards, the Bavarian Elector Maximilian believed that the war could no longer be won by military means and urged Emperor Ferdinand III to conclude a separate peace with France.

Philippsburg Fortress, named after Speyer's Prince-Bishop Philipp Christoph von Sötern, had been built in the former village of Udenheim because the bishop felt threatened by his Protestant neighbours, the Margraviate of Baden-Durlach and the Electoral Palatinate.

[2] Soon, Prince-Bishop and Archbishop-Elector of Trier Philipp Christoph fell out with Spain which occupied parts of the Electoral Palatinate around Frankenthal to secure the Spanish road, its critical military route to the Netherlands during the Eighty Years' War.

While Ehrenbreitstein was successfully handed over, Philippsburg's commander Caspar Baumberger refused and continued to hold the fortress in the name of Emperor Ferdinand II.

[6] The next year, Mercy went into the offensive, retaking the previous French conquests Überlingen at Lake Constance in May and Freiburg im Breisgau in July.

The French army, rebuilt by the Vicomte de Turenne, was first able to counter Mercy when reinforced by the Duc d'Enghien at the end of July 1644, few days after the fall of Freiburg.

[9][10] There he awaited Imperial reinforcements by Count Hatzfeld[11] and sent a cavalry detachment with more than 1000 men under Johann von Werth to interfere with d'Enghien's plans.

[9][8] From Philippsburg, French troops crossed the Rhine and took most hostile garrisons on the left bank, including Worms, Oppenheim, Mainz and Landau but not the Spanish Frankenthal.

[13] Although Mercy recovered Mannheim and Höchst in late autumn 1644 and cleared the right bank of the Rhine from French garrisons except Philippsburg,[14] his sovereign Maximilian of Bavaria was convinced afterwards that the war could no longer be won militarily.

[2] Philippsburg, conquered again in 1688 and 1734, was considered by Carl von Clausewitz to be a "model of a badly situated fortress" because it was too far from the Rhine to benefit from its proximity, but still close enough for the river to limit its effect.