Werth wrote about France in the prewar period and he also wrote about Russia in World War II, especially the Battle of Stalingrad and the Siege of Leningrad. He was one of the first outsiders to be allowed into Stalingrad after the battle. He spoke and wrote both Russian and English fluently. Werth spent most of World War II in Russia as a BBC correspondent, and had unrivalled access due to the combination of his BBC press credentials and his ability to function as a native Russian speaker. In January 1944, he was part of the delegation of Western correspondents who visited the graves in Katyn forest at the invitation of the Soviets. He believed the Soviet version that the Germans were the perpetrators. He was the Moscow correspondent for the Guardian newspaper from 1946 to 1949. His best-known work is "Russia at War, 1941 to 1945", (London, 1964) a behind-the-scenes look at life in the wartime Soviet Union. Other works include: "France 1940-1955: the de Gaulle Revolution"; "Moscow 41"; "The Last Days of Paris: a Journalist's Diary"; "Leningrad"; "The Year of Stalingrad"; and "Musical Uproar in Moscow."