Battle of Jena–Auerstedt
When Napoleon invaded Germany in 1806 and war broke out between the French Empire and Prussia, William supported his Prussian relatives, though he was nominally a French vassal. He received command of a Prussian division which took part in the Battle of Jena–Auerstedt. The Prussians lost that battle and William was forced to surrender his troops rather ignominiously at Erfurt the day after the battle. He was made a prisoner of war, but was paroled soon. Napoleon punished him for his betrayal, however, by taking away his principality. As a parolee, William was not allowed to take part in the hostilities anymore. After the Peace of Tilsit William received a pension from France in compensation.