One of the crucial failures of the Second Spanish Republic was in maintaining a sense of law and order. With the victory of the Popular Front, many of the Anarchist and Socialist movements began organized campaigns of theft, intimidation and murder against their political opponents.
The response of the Popular Front was largely to let it happen. What this did was radicalize the center - not to become fascist, but to see the government as no longer legitimate.
This view spread abroad as well. By mid-1936, the electoral shenanigans (forced re-votes that brought the Popular Front from a narrow majority to a 2/3rd one), systematic campaigns of church burnings, lynchings and land seizures gave the impression that the rule of law was completely lost in Spain.
The final straw was of course the murder of center-right politician Jose Calvo Sotelo. It wasn't that a leading legislator was dragged out of his home at night and shot to death - it was that it was done by the police with impunity.
The halting, half-hearted conspiracy led by disgruntled generals was instantly galvanized into concrete action by the deed. Fence-sitters like Fransisco Franco now realized that it was only a matter of time before they themselves were shot to death. Better to die fighting.
At this point, the government's legitimacy was gone. Everyday people who just wanted to get on with their lives and who otherwise took little interest in politics now pined for some relief from the growing fear and disorder.
As I pointed out in Long Live Death, the blow against the government's reputation was so powerful that instead of following established international law and allowing the recognized government to import arms, the League of Nations declared an embargo against both sides.
I mention this because a great many people think that foot-stomping and chanting "rule of law" somehow absolves lawless actors of their deeds. It doesn't. Even duly-elected governments are expected to conform to certain standards of justice. The more these standards are perverted, the less legitimacy remains.
The great mistake of the Republic was not only in abandoning those standards be in flaunting their abandonment. Calvo Sotelo's murderer wasn't even subjected to a fake trial. Under the existing constitution (drafted by the left, by the way), sitting legislators had immunity from arrest. Thus the crime blasted through every theoretical safeguard.
The message was obvious: "Even this person, protected with a special degree of legal immunity, can be cut down in cold blood. No one is safe."
Was it not understandable that some people would then move to use their own extra-judicial means to guarantee their personal security?
It's also worth pointing out that July 19 Rising was originally claimed to be in defense of the Republic, not to overthrow it. That came later. This was a solid rhetorical move, because the Popular Front had no easy way to reply. The Republic was dead, and they had destroyed it.
Historian Stanley G. Payne has repeatedly pointed out that the Popular Front didn't have to do what it did. There were many opportunities for them to check their actions and these would likely have helped them both consolidate their power and avoid imminent conflict.
However, I think there's something about the rush of power that comes from openly flouting the rules with impunity that quickly becomes addicted. Add to the fact that unstable people quickly gravitate to a movement that they think will allow them unlimited license and pretty soon all the guardrails will be removed.