The Spanish Civil War (1936-1939) is often seen as a precursor to World War II due to several key factors:

1. Proxy War: The conflict was a battleground for ideological warfare between fascism and communism. The Nationalists, led by Francisco Franco, received significant support from Nazi Germany and Fascist Italy, while the Soviet Union and international brigades from various countries supported the Republicans.
2. Testing Ground for Tactics and Technology: Hitler and Mussolini used the war to test new military tactics and technologies, including blitzkrieg strategies and aerial bombardment. This provided valuable combat experience and insights that were later used during World War II.
3. International Involvement: The Spanish Civil War drew in numerous foreign fighters and observers, foreshadowing the global nature of the upcoming conflict. Notably, the involvement of American volunteers in the war resulted in early confrontations between Americans and Nazi forces.
4. Polarization of Europe: The war heightened political tensions across Europe, solidifying alliances and enmities that would later play crucial roles in World War II. It starkly illustrated the coming global struggle between totalitarian regimes and democratic nations.

In summary, the Spanish Civil War was a microcosm of the ideological, military, and geopolitical conflicts that would erupt globally in World War II.