I think a good point to make about the Spanish Civil War is the fatal paradox of the Republic. The paradox theory (see: Antony Beevor’s book on the SCW) is this: the Spanish government elected in 1936 basically refused to arm the workers that had voted the government in and thus made it harder on its own supporters to help defend the popular front government.
Other points to make could be that Franco wasn’t the initial leader, instead veteran general Jose Sanjurjo was to be the new “caudillo” and general Emilio Mola led the main attack on Madrid (which failed, and ended up discrediting him). Only after the failure of Mola’s attack on Madrid did Franco emerge as he and his troops had defeated the republican siege of the Alcazar of Toledo.
Certainly also go with the Civil war within the civil war as communists and anarchists clashed in Catalonia in 1937 (possibly on Moscow’s orders). And the endless waste of manpower and materiel because the communists kept trying for major propaganda victories and kept failing against Franco’s air- and artillery superiority.
An interesting character is Soviet commissar Alexander Rodimtsev who ended up commanding the 13th Guards rifle division and as such reinforced Chuikov’s 62nd Army in Stalingrad. Of particular interest is that Rodimtsev had witnessed the bitter street fighting during the battle of Teruel (winter 37-38) when temperatures reached record lows in Spain. Teruel was interesting because the Republicans besieged Teruel, took the city and were immediately besieged themselves by the nationalists.
Without the betrayal of Chamberlain and Daladier (by signing the Munich agreement in Sept 1938) some people believe (I am among them) that the Republic could have easily held out until the outbreak of war in Europe in 1939. But because they were left to fend for themselves they (mostly the communists) decided on a fatal and futile offensive across the Ebro river, which was basically the end.
And the Spanish Civil War saw the first terror bombing of civilians, during the bombing of Guernica by the Condor Legion (commanded by Richthofen, who also commanded the German airforce at Stalingrad).
While I am not sure this really happened, I also would point at a story where Hitler sent admiral Canaris (of the Abwehr) to Franco to help convince Franco to join the war, and Canaris ending up deliberately doing the exact opposite.