The Battle of Rheinfelden was in fact two battles fought in 1638 to the north and south of the present-day town of Rheinfelden between a mercenary army led by Bernhard of Saxe-Weimar and an imperial army led by Johann von Werth. Bernhard was defeated in the first battle but managed to defeat and capture Von Werth in the second.
Following the Swedish defeat at the Battle of Nordlingen in 1634, Bernhard's mercenary army had come under the pay of France. Having been pushed back over 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 General Gallas in 1636.
Early in February 1638, having been prodded by the French government Bernhard advanced his army to the Rhine in order to find a crossing. Arriving at the town of Rheinfelden (an important crossing point) Bernhard prepared to invest the town from the south and using the ferry at Beuggen threw troops across the river in order to complete the investment from the north. The attack on the town was to be made on March 1.
In order to prevent this the Imperialists under the Italien Mercenary Count Savelli and Johann von Werth moved through the Black Forest to attack Bernhard's army and relieve the town.