Manfred (Moses) Stern (Russian: Манфред (Мойше) Штерн; also known as Emilio Kléber, Lazar Stern, Moishe Stern, Mark Zilbert; 1896–1954) was a member of the GRU, Soviet military intelligence. He served as a spy in the United States, as a military advisor in China, and gained fame under his nom de guerre as General Kléber, leader of the International Brigade during the Spanish Civil War.

Stern was born into a Jewish family in present-day Chernivtsi Raion, Chernivtsi Oblast, in western Ukraine on the border with Romania, which was at the time in the Duchy of Bukovina, a province of Austria-Hungary. He studied medicine at the University of Vienna.

Drafted into the Austro-Hungarian Army at the beginning of World War I, he was captured by the Tsarist forces and taken to a prisoner of war camp in Siberia. Freed after the 1917 October Revolution, he became a Bolshevik and joined the Red Army. He then led a partisan unit in Siberia against the White Army of Admiral Alexander Kolchak and fought in Mongolia against the warlord Roman von Ungern-Sternberg and his ally, the religious leader Bogd Khan. In 1921 he was elected to the Constituent Assembly of the short-lived Far Eastern Republic.

After the end of the Russian Civil War in 1922 he returned to Moscow and enrolled at the Military Academy. Upon graduation, in 1924, he joined Walter Krivitsky (also a Jew from Galicia) in the Red Army's Fourth Department, which was in charge of military intelligence (and which later evolved into GRU). Stern was initially assigned to the Comintern and acted as an instructor in its military schools.

In 1923, Stern, acting in the role of military advisor to Albert Schreiner was responsible for suggesting to Schreiner that Hamburg could be used as the first staging post for the communist insurrection.

In 1929, Stern became the GRU's chief spy in the United States. Based in New York City and operating under the cover name of Mark Zilbert, he managed a network of sources and agents involved in the theft of military secrets. In one operation they stole the plans for a new American tank. Another operation was foiled by a source who went to the American Naval Intelligence and then continued to deliver fake documents to the Soviets.^{[citation needed]}

The New York spy cell operated a safe apartment on West 57th Street, owned by Paula Levine, later part of a Soviet spy ring in Paris, and kept a photographic studio on Gay Street in Greenwich Village. There "Charlie," in actuality Leon Minster, GRU operator of a front, the Ellem Radio Equipment Shop, microfilmed the stolen documents. German sailors acted as couriers to the GRU in Europe. (These details come from Witness, the 1952 memoir of Whittaker Chambers.)

After handing off to Alexander Ulanovsky in New York, Stern traveled in 1932 to Shanghai where he served as the Comintern's military advisor to the newly created Jiangxi Soviet. Stern's activities in China remain veiled in mystery. In a report to the Moscow Comintern, he claimed that he tried to forge an alliance between the Chinese Red Army and a rebel Nationalist army whose officers had seized control of nearby Fukien province. However, this alliance failed and the National Revolutionary Army, under the command of Chiang Kai-shek, encircled the Chinese Red Army, forcing them to abandon their base in Jiangxi and to begin the Long March.