Violence against noncombatants

It was dangerous to be a civilian supporter of any group during the Spanish Civil War. Before the fighting broke out in 1936 there were a number of provocations by just about every party that took part in the war, but deaths were relatively uncommon. They largely occurred in breaking up strikes, assassinations, and other such actions. Once the war broke out, violence became much more commonplace. Paul Preston's The Spanish Holocaust is the best book on the subject and it describes the roughly 200,000 civilians killed during the war (roughly equal to combat casualties among combatants). Of those, roughly 50,000 occurred in the Republican zone. A further 10,000 people were evacuated/exiled from Republican territory.

Vocal supporters of what was considered the "other side" were in jeopardy almost immediately from the beginning of the war. On both sides, non-governmental groups armed themselves or were armed by the government. Few of these groups had much in the way of strict discipline, especially regarding distinctions between combatants and noncombatants. These groups ranged from unruly paramilitary (or ad hoc military) units to chaotic mobs.

From the instant the war started (due to a military coup), having a number of different statuses could cost you your life. In the Nationalist zone, being a union member, teacher, mayor or other elected politician, journalist, communist, anarchist, or a member of a militia could result in your murder. These were both spontaneous acts motivated by prewar rhetoric and a deliberate policy. In large swaths of the Republican zone, being a priest or clergy, outspoken Catholic, a businessman, a military officer, a large landowner (or landowner in general), a member of a rival faction from the left, or a member of other groups could result in your murder. These included spontaneous acts motivated by prewar rhetoric, but by and large the Republican government eventually attempted to stem the violence.

Violence against clergy

I do know that during the civil war, the Church openly supported Franco, and a number of priests were killed and churches were demolished

At this point we need to be careful with our terms. The Spanish Catholic Church—especially the hierarchy—largely supported Franco’s Nationalists. The Vatican and Catholics worldwide were much less ardent in their support. There wasn’t a schism between the Spanish Catholic Church and Rome, but the positions taken by Spanish bishops and cardinals were unintelligible to American Catholics, for example. This support by Spanish Catholics for Franco’s regime was criticized for its actions both at that time and later. Jose Sanchez in his book The Spanish Civil War as a Religious Tragedy has the following analysis:

In the final analysis, given all of the circumstances of the war and its background, Catholic support for the Nationalists was natural and logical. But was it necessary? Probably not. The Nationalists could never have afforded to antagonize or alienate the clergy and Catholics, who, after all, were their main base of support ... the clergy could have moderated the violence. They supported the Nationalists, but this did not mean they had to agree with everything the Nationalists did. (Sanchez 115)

Most of the violence against the Church during the Spanish Civil War was concentrated in anarchist-held regions of Republican Spain. In total, 6,832 clergy were killed. In addition, roughly 10,000 Churches were assaulted. Further, “the anticlericals ... tortured, they profaned, they burlesqued sacred ceremonies, they were violently iconoclastic,” (Sanchez pg 42), desecrated the tombs of nuns (pg 43), and “[p]eople stuck cigarettes in the corpses' mouths and mocked the mummies. Some even performed impromptu dances with the withered corpses ... In the church of San Antonio de Florida in Madrid the mob played soccer with the patron saint's skull.” Thousands of clergy were exiled from Spain as well. So, it wasn’t just murdering priests and religious—it was more widespread than that. Andrés Nin, a Spanish Communist leader, proudly stated during the war that “[t]he working class has solved the problem of the Church very simply; it has not left a single one standing.” The violence was widespread, ongoing, and planned.

Much of the violence was carried out by anarchists. The fact that the town of Barbastro hosted Durruti (a famous anarchist leader) and the town lost 123 out of its 140 priests is not a coincidence. However, it must be remembered that there was a general breakdown of law and order. The Nationalists were basically an augmented military coup, and the Spanish government was shaken badly by the violence. Some of the murders of clergy can be attributed to non-ideological killers. For instance, there were many prisoners that were freed just after the fighting began—including thieves and murderers. The logic that the state was the enemy of the people and that prisoners were victims of the state does have some appeal, but the cost was letting some truly vile people out into the populace. Some of these people killed anyone that had anything they wanted, and some of those victims were clergy. We also have to be careful in our attributions of blame, as pinning all of the violence on anarchist militias is unfair. Particularly those militias that were at the front had little opportunity to kill clergy behind the lines. There were also those who wanted to prove their loyalty to the Republic, and one way of doing so was committing violence against the clergy (This was mirrored on the Nationalist side by leftists proving their credentials to the new regime by joining the Falange and killing fellow leftists).

The level of violence against clergy was unprecedented.

The anticlerical fury of 1936 ... was the greatest bloodletting in the entire history of the Christian Church. (Sanchez, 8)

Also while its actions do not excuse the murder of its adherents, the Catholic Church in Spain had many structural problems that legitimately exacerbated the situation in Spain. There were economic problems that the Church largely didn’t address or even made worse, too many clergy that abused their positions for their own gain, and widespread antagonistic practices such as propaganda against the left based on The Protocols of the Elders of Zion advanced by Fr. Tusquests and others (that had their offical Catholic imprimatur removed by Cardinal Eugenio Pacelli in the Vatican). Anticlericalism was deeply rooted in Spain.

The dominant fact is that violent anticlericalism runs deeply throughout modern Spanish history, and this anticlericalism was caused by clericalism, or by perceptions of clericalism, and by very specific circumstances. (Sanchez, 21)

There were three main roots of anticlerical violence according to Sanchez.

1. the clergy perceived as military enemies in the specific circumstances of the uprising and war; 2) the clergy perceived as part of the old regime, to be destroyed for the aims of the revolution; 3) anticlerical violence perceived as part of Spanish tradition.

Practicing Catholicism in Republican Spain

Was someone endangering their self if they were open about being Catholic in Barcelona or Madrid under republican governance?

Only in the Basque territories were Catholics fully able to publicly practice their faith. The priest who swore in the Basque President Jose Antonio Aguirre had to flee for his life from Madrid in order to do the service, a service that was banned in most of Republican Spain. Churches were seized by Republicans for a number of uses. These ranged from the completely understandable needs of a war (setting up hospitals, group kitchens, even defensive positions in the thick of the fighting) to questionable uses (workshops, factories, repair depots) to deliberate provocations (brothels, latrines). So instead of going to your local parish you had to meet clandestinely.

For that, you needed a priest. Those who were not killed or imprisoned largely went into hiding (again, the exception being the Basque territories).

Some [priests behind Republican lines] were hidden by friends and relatives, some put on secular garb and melded into the general populace, but most were determined to exercise their priestly ministries by bringing the consolation of the sacraments to the laity, who they felt needed them now more than ever. (Sanchez, 60)

[T]he clandestine Church functioned on three different levels: in the prisons, in the embassies, and among the general populace (Sanchez, 62)

[I]n the prisons there was little need for clandestine activity (Sanchez, 62)

So to practice the Catholic faith (and to receive its sacraments) you had to go underground. The other option was to get a foreign diplomat to grant you sanctuary, which 15,000 people were able to do during the war.

Catholic support for the Republic

Were there any Catholics fighting for the republic, including international volunteers?

This was rare, at least for those who would self-identify as Catholic. The majority of international Catholic volunteers would be the 600 Irish volunteers under Eoin O’Duffy—and these fought on the Nationalist side). Two Cardinals refused to sign the letter of support for Franco (Múrgica and Vidal i Barraquer) drafted by Cardinal Gomá. A few prominent priests backed the Republicans (Sanchez gives a mere three examples).

Conclusions

The Spanish Civil War was a tragedy on many levels. In my opinion chief among them was the violence against noncombatants on both sides. The Catholic Church suffered an unprecedented amount of violence as well as a great del of repression. None of that happened in a vacuum. The books by Sanchez and Preston go into much more detail needed to fully understand the situation, but followup questions from OP and others are encouraged.