Although the Spanish Civil War was clearly internal in its origin, the international situation played a decisive role in the duration, progress and final result of the conflict. The rearmament policies followed by the principal countries of Europe since the beginning of that decade created a climate of uncertainty and crisis that undermined international security. The Soviet Union began a large-scale programme of military and industrial modernisation that was to position it as the leading military power over the next few decades. At the same time, the Nazis, under Hitler, committed themselves to overturning the Versailles agreements and restoring Germany's dominance. Mussolini's Italy followed the same path, and its economy was increasingly devoted to preparing for war. France and the United Kingdom began rearming in 1934, and this process escalated after 1936. The world arms trade doubled between 1932 and 1937. According to Richard Overy, ‘the popular antiwar sentiment of the 1920s gradually gave way to the reluctant recognition that major war was once again a serious possibility’.

Under these circumstances, none of these countries showed any interest in stopping the Spanish Civil War. International support for both sides was vital for fighting and continuing the war during the early months. Italian and German aid enabled the military rebels to move the Africa army to the peninsula at the end of July 1936, and Soviet aid made a decisive contribution to the republican defence of Madrid in November 1936.