Why Hitler bombed four peaceful villages in Spain?

Spanish Civil War, May, 1938. Four villages in Castellón, Benassal, Albocàsser, Ares del Maestrat and Vilar de Canes, were bombed from the sky and ravaged. 38 people died. Inhabitants never knew for sure who piloted the planes responsible for such atrocity, although the rebel propaganda attributed the act to the republican side. Now, 80 years later, the truth is finally exposed.