But what did the International Brigades actually achieve?
First, the achieved the prolongation of a nasty, brutal war.
Second, by that first achievement they ensured the deepening of an intense enmity that still lasts to this day.
Third, They provided a training ground for the German National Socialists to home their battle plans, including the use of air power.
Fourth, they ensured that a rival socialist group, the Marxist/Stalinist communists, would become defined as the de facto opposition to National Socialism.
Fifth, they created a situation where the Moscow inspired Commissars were, effectively, given a free hand to kill off anyone who appeared to offer a threat to the supremacy of Moscow. It is said that the Commissars in Spain killed more Italian Communists than Mussolini himself.

I do not say the above to denigrate the undoubted courage, bravery and determination of those men and women who made up the International Brigades, but merely to suggest that, by naively interfering in a foreign war, they may have made matters worse.