In 1936, under the name General Walter, he was sent to Spain during the Spanish Civil War, where he initially led the XIV International Brigade in the Battle of Lopera, and later the 35th International Division in the Segovia offensive, and the battles of Brunete, Belchite, Teruel, the Aragon Offensive and the Battle of the Ebro. By April 1938 Spanish communist leaders wanted the replacement of many International Brigade commanders due to poor performance, and although André Marty disagreed, he had to compromise and General Walter and Vladimir Ćopić were replaced.