Donald Duart MacLean was a British diplomat who was recruited during his studies at Cambridge by the Soviet intelligence service due to his known communist/Marxist sympathies. Prior to World War II, he worked at the British embassy in Paris, where he was privy to information about British diplomacy with France and Germany; he passed that information on to his Soviet handlers. In the latter half of the 1940s, he was transferred to Washington DC, where his spying provided the Soviets with key information about American thermonuclear policy and capabilities.
He was eventually appointed head of the American department in the Foreign Office, but by then, he was widely suspected of being a spy. His Soviet handlers ordered him to defect in 1951, and he fled to Moscow, where he quickly assimilated and spent the next few decades working as an expert on Soviet-NATO relations and British policy.