Encouraged by his agent, Gill Coleridge, I had read Callum's earlier book on the Korean War, which impressed me as a very clear and fair-minded account of a difficult subject. Callum brought to this, as to all his other books, a cool and judicious intelligence, high standards of scholarship, and penetrating powers of analysis. When I met him for the first time, I was slightly surprised to find him a youthful, restless, somewhat scruffy figure who spoke with a strong and occasionally incomprehensible Glaswegian accent.
There was not a shred of pomposity about Callum. It was typical of him that he was inspired to write his Heydrich book by a war memorial he found in his local park. Though I knew he deserved the post, I could never quite reconcile myself to the idea of Callum as a professor - in his appearance and in his behaviour, he seemed more like an undergraduate. What impressed me most was his enthusiasm. He was never less than excited by what he was doing. Conversation with him was a bit like boarding a spinning carousel; you got the impression he had started before you arrived and you had to keep your wits running to jump aboard.