Query:  What river did the French cross to engage the Prussian army?
Context: As in previous campaigns, his fundamental objective was to destroy one opponent before reinforcements from another could tip the balance of the war. Upon learning the whereabouts of the Prussian army, the French swung westwards and crossed the Saale with overwhelming force. At the twin battles of Jena and Auerstedt, fought on 14 October, the French convincingly defeated the Prussians and inflicted heavy casualties.