The Spanish Civil War was the most broadly mobilized and militarily innovative of the European civil wars, and proportionate to population was fought on the largest scale. The Russian conflict lasted longer, was fought over a much greater geographic expanse, and at one time or another involved many more men and women, but, despite calling up more than 5 million men, at no time did the Soviet Red Army have more than a large fraction of them fully mobilized and ready for combat. Before the end of 1937, both armies in Spain had organized a greater percentage of their zone's population, and also had armed them better.

Nonetheless, neither contender in Spain could be compared with the armies of the major powers in the world wars. Even though more than 2 million men were called up by both sides combined, there were severe limits to training, organization, and leadership. Front lines were extremely long and offensive operations were narrowly focused, leaving most sectors quiescent. Overall, the Spanish struggle was a low-intensity conflict punctuated by a number of high-intensity battles. Despite the introduction of late-model planes and antiaircraft guns, firepower was in general limited, which explains why casualties were not higher. Not many more than 175,000 men were killed on both sides combined (some 25,000 of them not Spanish), slightly less than 6 percent of all mobilized. This was a significant figure, yet there could be no comparison with the catastrophic loss of life in the Russian civil war. Millions of Russian civilians perished because of very poor nutrition, bad sanitation, and massive epidemics; such scourges were much more effectively controlled in a West European country like Spain. Medical care on both sides was much better than in the Russian civil war, with many fewer deaths from wounds, particularly as a result of techniques pioneered in the People's Army by the radically innovative Dr. Josep Trueta, who developed new surgical procedures for puncture wounds.