 Complete the following phrase:  Vian specialised in naval gunnery from the end of World War I, and subsequently received several appointments as gunnery officer. In the early 1930s, he was given command of a destroyer, , and, later, various destroyer flotillas. During this phase of his career, in early 1940, he commanded a force that forcibly released captured British merchant sailors from the German supply ship Altmark in Jøssingfjord in then-neutral Norway and, later, his flotilla took an active role in 