Why is the Spanish Civil War often referred to as a “dress rehearsal” for the Second World War?

The Spanish Civil War can most accurately be described as the dress rehearsal for the Second World War as it had a direct impact on Soviet foreign policy, and helped lead to the final sequence of events that sparked World War Two. Lack of intervention from western democratic nations in the Spanish Civil War left support of the Second Spanish Republic largely to the Soviet's, showing Stalin that the nations that went on to become the allies weren't committed to protecting democracy from fascism. This directly led to the Soviets choosing to isolate themselves from the allies and instead pursue cooperation with Nazi Germany, in the form of the Molotov-Ribbentrop pact being signed shortly after the Spanish Civil War. This mutual non aggression pact was immediately broken by Nazi Germany as they invaded Poland on 1 September 1939 starting the Second World War.

encyclopedia.ushmm.org/content/en/article/german-soviet-pact pure.qub.ac.uk/en/publications/soviet-foreign-policy-from-the-spanish-civil-war-to-the-molotov-r