Opening during the Spanish Civil War, the 1937 Paris Exhibition allowed artists to speak out against brutality. Fiona Macdonald looks at a moment when paintings became propaganda.

On 26 April 1937, Nazi German and Italian bombers attacked the Basque city of Guernica. Over the course of three hours, they destroyed three-quarters of the ancient town, killing and wounding hundreds. The raid was “unparalleled in military history”, according to reports at the time – and it inspired one of the most famous anti-war paintings in history. A new exhibition staged in London by Barcelona’s Mayoral Gallery honours a group of artists who responded to the atrocities of the Spanish Civil War.

These artists were brought together by the 1937 Paris Exhibition, which opened less than a month after the bombing and just 10 months after the Civil War began. The Exhibition is usually remembered for the competing bluster of two nations: Germany, with its monumental granite tower topped with a giant eagle and swastika, and the Soviet Union, whose marble-clad structure was capped by an even bigger statue of two figures clutching a hammer and a sickle. Yet it also played host to a humbler project that has outlasted either monolith. Mayoral’s exhibition commemorates the 80th anniversary of the Spanish pavilion, seen by the Second Spanish Republic as a way of revealing General Franco’s cruelty to the rest of the world against a backdrop of rising authoritarianism.

Its ambitions were far removed from Nazi and Soviet architectural one-upmanship. As Europe moved towards war, the situation in Spain took on significance around the world. It became a battleground for the forces of Fascism and Communism and inspired new works from some of the greatest artists of the time. Pablo Picasso, Julio González, Joan Miró, Alexander Calder, Alberto Sánchez, and José Gutiérrez Solan were all shown in the Spanish pavilion.

Picasso was commissioned to create a mural for the pavilion, and had started on a series of anti-Nationalist images called Dream and Lie of Franco earlier in 1937. After reading reports of the attack on Guernica by Franco’s allies, he began work on a painting that would come to symbolise the wider fight against Fascism. According to the art historian Fernando Martín Martín, “For the first time in the contemporary history of war, a town and its civilian population had been annihilated both as a scare tactic and a way of testing the war machine.” He says this was the “instant Picasso knew what would be the subject of his mural for the pavilion.”

In Guernica, I clearly express my abhorrence of the military caste which has sunk Spain in an ocean of pain and death – Pablo Picasso

His painting, Guernica, is not on display at Mayoral (it is exhibited at Madrid’s Reina Sofía Museum) – but there are insights into its creation, including photographs taken by Picasso’s girlfriend at the time, Dora Maar. The mural took him just over a month to complete. While painting, to combat rumours that he supported the Nationalists, Picasso issued a statement: “In the panel on which I am working, which I shall call Guernica, and in all my recent works of art, I clearly express my abhorrence of the military caste which has sunk Spain in an ocean of pain and death”.

Political canvas

The artists creating pieces for the pavilion were explicit in their aims. Mayoral’s exhibition curator Juan Manuel Bonet says that “all the major works at the pavilion were fruits of a commission. The special thing about this commission was that it was not intended to be a political commission; the artists took it upon themselves to react in such a way.”

It was a new outlook for some of them, says Bonet. “Before 1936, neither Picasso nor Miró were very political; but the Spanish Civil War changed this.” According to him, 1937’s Dream and Lie of Franco by Picasso and Aidez l’Espagne (Help Spain) by Miró are the artists’ first overtly political works. “Later on in their careers, Picasso joined the French Communist Party in 1944 and Miró continued to be very active against Franco’s regime into and during the 1960s and ‘70s.”

The American sculptor Alexander Calder’s contribution to the pavilion was also a piece of propaganda. A supporter of the Republican cause and great friend of Miró, Calder was initially refused permission to create an artwork because he wasn’t Spanish, but the organisers relented after a marble fountain from Spain had to be repurposed. It was filled with mercury that was poured through a series of sculptures created by Calder until it reached a mobile labelled ‘Almadén’.

The word resonated with Republicans. It was the name of a stronghold that held out against an offensive by Franco’s troops in March 1937, famous for its deposits of mercury, an element valued for its use in manufacturing weapons. Calder’s piece, The Mercury Fountain, doubled as a symbol for Republican resistance. “Nothing in the pavilion was free from intention,” writes Martín.

Guernica was not a picture but graffiti, though graffiti done by a genius – José Bergamín

The exhibition has been put together with Joan Punyet Miró, historian and grandson of Joan. It includes a reconstruction of his grandfather’s El Segador (The Reaper) – a mural painted onto construction material in situ which was then lost or destroyed after the pavilion was dismantled. Showing a Catalan peasant with a huge misshapen head, it was a cry of outrage at the events in Spain. “Of course I intended it as a protest,” said Miró. “The Catalan peasant is a symbol of the strong, the independent, the resistant.”

Pop-up propaganda

Both The Reaper and Guernica were created as propaganda, in the manner of Soviet agitprop – “ephemeral art based on propaganda and agitation for a political cause aimed at stirring up the masses”, writes Joan Punyet Miró, arguing that the murals “looked like huge political propaganda posters”. The poet José Bergamín commented that “Guernica was not a picture but graffiti, though graffiti done by a genius”.

Painted on poor quality canvas, Guernica could easily have been destroyed as well. According to Punyet Miró, “Neither of them chose a tough, hard-wearing support, for they knew in advance that these were ephemeral works, designed to cause an impact and then disappear along with the pavilion… Guernica was spared the same fate as Miró’s mural because Picasso was asked to send it to London and later to the United States”.

The painting was not to return to Spain until democracy had been returned – Juan Manuel Bonet

As it turned out, the pavilion was only the beginning. Guernica toured around the UK in 1938, says Bonet. “Picasso later entrusted the painting to MoMA in New York, as it was his wish that the painting not return to Spain until democracy had been returned to the country. This was symbolically very important.”

Through a dark lens

Guernica took on a wider meaning in the years that followed. “It speaks about the Spanish Civil War, and the destiny of civilians in it, as well as the bombs that killed so many people in this Basque city,” says Bonet. “It also remarks on all wars.” The French writer Michel Leiris was moved to say of it: “On a black and white canvas that depicts ancient tragedy… Picasso also writes our letter of doom: all that we love is going to be lost.”

Yet there was hope in it too. Amid shrieking figures and corpses, Picasso left a beacon, according to Martín. “At the top, stretching out from a window, a woman with an oil lamp seems to want to illuminate the encroaching panic and darkness.”

Guernica tapped into an earlier tradition, echoing Goya’s works commemorating resistance to the Napoleonic invasion of Spain. According to curators at the Reina Sofía, “The grotesque vision that Goya brought to his political critique was not lost on artists as a powerful tool for crafting their own views of the present.” Picasso admired Goya’s “dark lens on Spain’s complex political and religious traditions”.

“Picasso, Miró, Calder and González taught us that sometimes major moments such as the Spanish Civil War force us to take sides,” says gallery director Jordi Mayoral. “The works created by these artists for the pavilion are still part of the Spanish collective memory; they represented a major turning point in the Civil War and the country’s struggle between democracy and fascism”.

Picasso himself summed up his decision, remarking in 1937: “I have always believed and still believe that artists who live and work with spiritual values cannot and should not remain indifferent to a conflict in which the highest values of humanity and civilisation are at stake.”

If you would like to comment on this story or anything else you have seen on BBC Culture, head over to our Facebook page or message us on Twitter.

And if you liked this story, sign up for the weekly bbc.com features newsletter, called “If You Only Read 6 Things This Week”. A handpicked selection of stories from BBC Future, Earth, Culture, Capital, Travel and Autos, delivered to your inbox every Friday.