What was one reason why the spanish civil war and the bombing of guernica was called a dress rehearsal for ww1?

The Spanish Civil War and the bombing of Guernica were considered a "dress rehearsal" for World War II because they showcased the tactics and technologies that would later be employed on a larger scale during the global conflict. The use of aerial bombardment, particularly by the German Luftwaffe in Guernica, demonstrated the devastating impact of modern warfare on civilian populations and infrastructure. Additionally, the ideological battles between fascism and communism in Spain mirrored the broader geopolitical struggles that would define World War II. This conflict also served as a testing ground for military strategies and alliances that would emerge in the subsequent war.