The Battle of Ocaña (or Ocana) was fought on 19 November 1809 between French forces under Marshal Nicolas Jean de Dieu Soult, Duke of Dalmatia and King Joseph Bonaparte and the Spanish army under Juan Carlos de Aréizaga, which suffered its greatest single defeat in the Peninsular War. General Juan Carlos de Aréizaga's Spanish army of 51,000 lost nearly 19,000 killed, wounded, prisoners and deserters, mostly due to the French use of their cavalry. The strategic consequences were also devastating, as it destroyed the only force capable of defending southern Spain; the area was overrun over the winter in the Andalusia campaign.