Lee:
It's an area where I don't have much knowledge. Can't even recommend good books. Not a very simple conflict either.
It was very complex indeed, it doens't "just happened" in 1936. It has very ancient causes as with the power of the Church on one hand and that of the aristocrats vs the growing discontent of popular sectors against the status quo. But it seems that the development of the First Republic was cut off by dictatorship and then the Second Republic was not absent of all kinds of influences... and when I say "all kinds: that means all: fascists, comunists, anarchists (a very strong movement, the storngest of that kind ever, social democrats, monarchist, "carlists", hiper catoclics, etc"
The protestant anglo saxon public opinion is severely favourable to the "republicans", specially because many intelectuals such as George Orwell or Hemingway participated on the Republican side and because Hitler supported the Nationalists. However things get more complicated here. By 1933 the right wing CEDA won popular elections and the leftist president then, Alcala Zamora, avoided to call to form a goverment and instead came with an institutional trick. By 1934 there was violence and the left started targeting eclesiastical personnel. The 1936 elections were the culmination of a whole situation, very complex. The right wing was also divided in rival factions: it seems that the natural leader could have been a very inteligent and strong guy called Primo de Rivera, which was feared by both, the left and Franco's nationalists. Rivera was the founder of the Spanish Phalanx and was executed at the very begining of the civil war while being in prison, where (like the Che in Cuba later) he became a martir of the cause. It seems that even Franco wasn't that prone to start the civil war until conditions presented themselves.
I still need to start reading Beevor which I know will be enlightining as with his book on Stalingrad. But it seems that he also mades an analysis on why the Republicans failed to win. This analysis avoids the western common knowledge that claims that Hitler won it for Franco, but goes in deep in the military problems the Republicans had, how they were unable to adapt themselves to the kind of warfare that developed in that conflict. At the end the Republicans bleed themselves to death at the Ebro, but like many battles, the Ebro was the physical evidence that the Republicans already lose the war.
Anyway the Spanish Civil War was the "appropiate" introduction to WWII. It foretold the violence and cruelty that Europe and Asia would witness later the following decade.