Based on a combination of a wide range of second-hand sources with previously unknown archival material from Spain, Britain, France and the United States, this book explores the Spanish Civil War of 1936-39 as a propaganda battle aimed mainly at foreign public opinion. It shows how both Nationalists and Republicans used the experiences of previous conflicts such as World War I, as well as that of their totalitarian allies, in order to set up a number of propaganda and censorship services with the goal of persuading foreign -- and specifically British -- audiences of the legitimacy of their causes, and of the need to give them political, military, and relief assistance. The propaganda messages designed by both sides -- ranging from the atrocities committed by the enemy to illegal foreign intervention on its behalf -- are analysed in detail, together with the techniques that were employed to transmit these messages: eye-witness accounts, official commissions, unofficial missions of investigation, documentaries, art exhibitions, etc. As to the impact of both campaigns on the British population, the author argues that their crude nature helped to mobilise both the extreme right and the extreme left, but alienated the great majority, who preferred to rally to the Non-Intervention policy adopted by the Baldwin and Chamberlain governments. The chronicle of this relatively neglected topic demonstrates not only the utter modernity of the Spanish conflict, but also the origin of some of the arguments still employed by current historians of the war.