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.
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.
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).
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.
There "Charlie," in actuality Leon Minster, GRU operator of a front, the Ellem Radio Equipment Shop, microfilmed the stolen documents.
[1][2]) 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.
In thinking about him it is hard not to ponder on the ironical fact that Hitler is not the only native of Austria who is playing a great part in the Spanish Civil War.
[7] The NKVD chief in Spain, Alexander Orlov, knew that Stern's recall meant certain imprisonment and death because in Moscow Joseph Stalin and Nikolai Yezhov were busy purging the Red Army.
While awaiting orders, Stern spent his final months in Spain relaxing at a small orange plantation and entertaining his young Spanish mistress.