Napoleon invaded Prussia with 180,000 troops, rapidly marching on the right bank of the River Saale. 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. With several major commanders dead or incapacitated, the Prussian king proved incapable of effectively commanding the army, which began to quickly disintegrate.