I doubt there is a "whom" considering it was pretty much common wisdom and propaganda from the day the war started. The foreign volunteers on the Francoist side though that they were fighting a global war against Communism, and the ones in the Republican side though they were fighting a war against Fascism. As the war came to an end there was a faction in the Republican govenremnt, led by PM Juan Negrín, that defended to keep up the fight until war broke out between the European powers, which they believed imminent. Negrín himself summed it as Resistir es vencer ("Resistance is Victory").