1936 Civil War broke out in Spain between the Republican Government and the Nationalists led by General Franco.
Stalin Suppored the Republicans by giving them weapons, aircraft and pilots.
Both Hitler and Mussolini declared they would support Francos forces as they shared similar views.
Unlike both USSR and Germany, Britian and France refused to intervine as they feared their involvement would start a large scale European war.
Mussolini sent thousands of troops and Germany sent both aircraft and pilots allowing them to perfect bombing of civilan populations.