Likely World War II, where the Germans (who aided the Nationalists, or fascists) again fought the Russians (who aided the Republicans, representing socialism).
But it was also prescient of the Cold War, because the US, Britain, and France withheld and/or embargoed aid to the Spanish Republic, because it included communist factions. In World War II, only the strategic advantage of a two-front war prompted extensive Allied military aid to the Soviet Union.
Both the Mexican War and Bloody Kansas have been referred to as the Dress Rehearsal for the Civil War.
The Spanish Civil War was a rehearsal for Germany to test out some of its weapons and leaders. The Germans sent the Condor Legion to fight in Spain.
The Spanish Civil War
Because the German Air Force practiced dive bombing in Spain during their civil war.
Full dress rehearsal
Dress Rehearsal - 1948 was released on: USA: 21 March 1948
Full dress rehearsal
Full dress rehearsal
The cast of Dress Rehearsal - 1948 includes: Richard Goode as The Director
The cast of Dress Rehearsal - 1956 includes: Eric Sykes as Director
There are thousands of types of rehearsals, ex. band, drama, singing, but the only named rehearsal is a dress rehearsal, which is the final rehearsal before a performance.
Full dress rehearsal