A reply to /u/yourlizardoverlord

Peter Wilson's book covers this battle in some detail. Following Imperial victory at Nordlingen, all active belligerents in the conflict entered into an uneasy situation with the threat of France looming. The French had paid a big part of the Swedish army's expenditure and they have also hired Saxe-Weimar's small army. The French prodded him to take his under strength unit to take the key town of Rhinefelden, which as the name suggests sits on the Rhine, which runs west-east south of the town. It had a small Imperial garrison.

Saxe-Weimar decided on a rapid attack knowing he was under strength and had little artillery. The initial speed worked on his favor but the garrison held. Thus a siege began. Imperial forces elsewhere realized what had happened and they hastened to help the garrison. They left their baggage and artillery behind and marched through a mountainous region. They were thwarted by Saxe-Weimar's Dragoons at first approach and had to march around taking an even longer route. Eventually they prevailed and largely routed Saxe-Weinar's army who appeared to flee south and east on the left bank of the Rhine.

Since the Imperials had marched a long ways (from Breisgau about 60km north), they didn't give chase and decided to rest. Saxe-Weimar didn't fully retreat, rather he marched east, crossed the river, and doubled back west on the right bank of the river. In early morning, he surprised the Imperials who were tired, scattered, and had almost no gunpowder left. Remember they had forgone their baggage elsewhere. So the rabble army fired one salvo and charged in. Imperial forces panicked and ran, except for a Bavarian battalion which held.

So it was a surprise comeback that fell upon an army that had exhausted itself on a forced March.