In what sense was the Spanish Civil War a "dress rehearsal" for World War II?

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The Spanish Civil War was fought between the nationalist forces, led by General Francisco Franco, who were supported by Nazi Germany and fascist Italy, and the Republican forces, supported by the Soviet Union and other leftist groups. This ideological divide and the involvement of major powers on both sides foreshadowed the global conflict of World War II, which was also driven by a clash of ideologies and involved the same major powers and alliances.

In What Sense Was the Spanish Civil War a "Dress