The flames of Guernica still burn in modern memory. In 1937, the Nazi German air force bombed this ancient city in the hills of Spain’s Basque region on behalf of the fascist side in the Spanish civil war. The attack took more than 1,600 lives and was a revelation of the horror of bombing raids on civilians, which would soon become the norm when world war broke out two years later.

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This week former minister Andrew Mitchell called up Guernica’s ghosts. “What Russia is doing to the United Nations is precisely what Italy and Germany did to the League of Nations in the 1930s,” he said in the House of Commons. “And they are doing to Aleppo precisely what the Nazis did to Guernica in the Spanish civil war.”

The power of Guernica – to make us see the crimes of our own time more clearly – is a tribute to the moral efficacy of art. There is a reason everyone recognises Guernica as an image of the barbarity of bombing. The second world war saw far more murderous air attacks, yet Guernica is the most universally remembered example of the pity of air raids – because Picasso painted it.

Mitchell referred to the bombing of the Basque capital as a comparison to wake us up. Portuguese cartoonist Vasco Gargalo, meanwhile, has returned to Picasso’s painting itself. His viral picture Aleppo inserts the faces of Vladimir Putin and Bashar al-Assad into Picasso’s nightmare and turns an impaled horse into an allegory of the United States.

Why can Guernica help us to see the truth about Syria? Picasso painted his masterpiece quickly on a huge panel in his studio in a loft on the Left Bank in Paris. He showed it at the Spanish pavilion in the 1937 Paris universal exhibition. It did not stop the horrors of the Spanish civil war nor save Spain from 36 years of fascist rule. Yet looked at today it is prophetic; every bomb dropped on a human being makes its profundity all the more apparent.

Guernica is not a conventional history painting nor a factual recording of what happened, but a cubist apocalypse painted by the most revolutionary of modern artists. The human body is no match for the technology of bombing, and Picasso shows us exactly what that means. A warrior’s remains lie broken and scattered. The dead face of a baby is delineated in a few simple heartbreaking marks. Someone is trapped in a burning building.

Facebook Twitter Pinterest A wounded Syrian child cries in Aleppo, Syria on October 11, 2016. Photograph: Anadolu Agency/Getty Images

Through the fragmenting lens of cubism he is able to make us see a battering tumult of horrors in the same instant. He forces us to respond to them as real, tangible, physical moments of incredible violence and pain. By abstracting from reality, Guernica concentrates the mind on the essence of atrocity: the dead baby’s face, the figure in the fire. It can make us see extremes of injury and death that most photographs rightly shun, because Picasso paints the human suffering within, rather than gazing on cruel details.

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In 1937 the courage of this painting was to tell the truth in an age of lies. That is still what it does. The most awful analogy between today and the 1930s is that truth is being crushed by lies and propaganda – we are in a post-truth age. Politicians such as Putin and Donald Trump say whatever they like. The internet muddies every fact with a counterfact and there are so many conspiracy theories that no one believes anything.

Yet we have been here before. In the 1930s, totalitarian propaganda machines attacked the very idea of truth. Hitler and Stalin waged permanent war on facts using radio, cinema, censorship and brainwashing mass rallies.

Picasso knew exactly what he was doing when he painted Guernica. He was trying to show the truth so viscerally and permanently that it could outstare the daily lies of the age of dictators. Nazi Germany and Stalinist Russia had huge pavilions at the 1937 exhibition where Guernica was unveiled, but Picasso – just one man bearing witness to the truth – painted the human reality of that vile time. It has outlasted all propaganda.

We need to fight for the truth as bravely as Picasso did. It is not only bombs that are killing people in Aleppo, but Putin’s lie machine as well.