On his release in August 1935, he travelled to Spain where in July 1936 he joined the International Brigades in support of the Spanish Republican cause, becoming Chief of Staff for Lieutenant Colonel Hans Kahle in the XI International Brigade and the 35th Division. He was in Madrid in November 1936 writing training booklets for the military command as the city became increasingly under threat. He was driven out of the city to Cuenca by Claud Cockburn, a British communist journalist, under orders from the government and their Russian advisors. In November 1936 he became leader of the Thälmann Battalion of the XII International Brigade, fighting in the Battle of Guadalajara in March 1937 and then in the Battle of Brunete. In July 1937 he attended the Second International Writers' Congress, the purpose of which was to discuss the attitude of intellectuals to the war, held in Valencia, Barcelona and Madrid and attended by many writers including André Malraux, Ernest Hemingway, Stephen Spender and Pablo Neruda. In August 1937 he was sent on a pro-Republican propaganda tour to the United States. During his service in Spain he spent time with Ernest Hemingway. He wrote an account of his time in his work, "Der spanische Krieg" (1955), but was unable to name Hemingway, referring to him only as 'an American', because of the ideological condemnation of Hemingway's work in the GDR at the time.