When Duke George William, Duke of Brunswick-Lüneburg inherited the Principality of Lüneburg, he appointed Josias as major general and put him in command of the much enlarged army of the principality. In late autumn 1668, Josias marched against the Turks with 3300 men, to defend possessions of the Republic of Venice. During the Siege of Candia, he was injured by shrapnel on 6 July 1669. On 29 July, he died of his injuries. He was initially buried in the St. Catherine's Church in Candia. His body was later transferred to Wildungen.