Complex sentence: In the First World War, Eder joined the British Army, serving in the Royal Army Medical Corps, as a temporary captain, he was appointed medical officer in charge of the psycho-neurological department in Malta.
Split into simpler sentences: In the First World War, Eder joined the British Army, serving in the Royal Army Medical Corps. As a temporary captain, he was appointed medical officer in charge of the psycho-neurological department in Malta.