Having been pushed to the west bank of the Rhine by the Imperial advance, Bernhard's army had settled in Alsace during 1635 and had done little except help repulse the Imperial invasion of France under the Cardinal-Infante Ferdinand and Matthias Gallas in 1636.

Early in February 1638, having been prodded by the French government, Bernhard advanced his army of 6,000 men and 14 guns to the Rhine in order to find a crossing. Arriving at an important crossing point at the town of Rheinfelden, Bernhard prepared to invest the town from the south.

In order to prevent this, the Imperialists, under the Italian mercenary Count Federico Savelli and German general Johann von Werth, moved through the Black Forest to attack Bernhard's army and relieve the town. Bernhard was beaten in the first battle but managed to defeat and capture Werth and Savelli in the second.