In 1806 the Fourth Coalition was formed by Great Britain, Sweden, Russia, and Prussia against Napoleon and France. Napoleon invaded Prussia in the fall. Charles William Ferdinand, Duke of Brunswick-Wolfenbüttel commanded the Prussians at Jena-Auerstadt. He was 71 years old while his field commanders were in their 60s. The Prussian army was still using tactics and training of Frederick II of Prussia. Its greatest weakness was its staff organization. Most of the divisions were poorly organized and did not communicate well with each other. Napoleon's army was experienced and well led. The armies met at Jena-Auerstadt on October 14, 1806.