 Clausewitz served during the Jena Campaign as aide-de-camp to Prince August.At the Battle of Jena-Auerstedt on 14 October 1806—when Napoleon invaded Prussia and defeated the massed Prussian-Saxon army commanded by Karl Wilhelm Ferdinand, Duke of Brunswick—he was captured, one of the 25,000 prisoners taken that day as the Prussian army disintegrated.He was 26.