What became a dress rehearsal for world war II by demonstrating the destructive power of modern warfare?

The Spanish Civil War (1936-1939) served as a dress rehearsal for World War II, showcasing the devastating effects of modern warfare. It involved extensive use of aerial bombardment, tanks, and new military tactics, highlighting the destructive capabilities of technology and ideology. The conflict drew in international powers, with Nazi Germany and Fascist Italy supporting Francisco Franco, while the Soviet Union backed the Republicans, foreshadowing the alliances that would shape the upcoming global conflict. This brutal civil war underscored the potential for widespread devastation that would later be realized during World War II.