Aragon is like a desert in autumn. Beside the road from Zaragoza, dust devils whirl like dervishes above a bone-dry plain speckled with desiccated ruins. Twenty miles south of the turn-off, a church tower rises from the monk-brown earth, its symmetry seemingly deformed by the haze. But the distortion is real, and the smashed houses that surround it, like broken teeth in a punched-out mouth, are even uglier than they looked in the old photographs. This is the ghost town of Belchite, and if any town ever looked better in black and white, this is it.

The last time the International Brigades volunteer Paddy Cochrane, 94, was here was in 1937. He was blown up and left for dead after a mercy mission that went badly wrong. Now he sits by me in the hire car, his expression swinging between anticipation and dread of revisiting the spot where he should have been buried. He carries, too, the faint hope that among the depleted ranks of fellow veterans meeting here to commemorate the battle’s 70th anniversary, he might find the man who risked his own life to haul him to safety all those years ago.

Three miles out we divert from the main road on to a dirt track so we can approach from the east. It was from this direction that Paddy first saw Belchite in September 1937, a cluster of burning dwellings and shell-shocked steeples that stubbornly, suicidally, refused to surrender.

The attack had gone startlingly wrong for the Republican forces. Belchite should have been no more than a brief diversion on the road to the rebel-held city of Zaragoza, the capital of Aragon and a significant prize for the Republic. But a heroic defence by fascist troops and local militia – the newly elected mayor died with a rifle in his hands – had stalled the advance for 13 days, and the smell of the unburied dead was so strong that troops took to wearing their gas masks.

Bill Bailey, a volunteer with the American Abraham Lincoln brigade, described the fighting. “We would knock a hole through a wall with a pickaxe, throw in a few hand grenades, climb through into the next house, and clear it from cellar to attic. And, by God, we did this hour after hour. The dead were piled in the street, almost a storey high, and burnt. The engineers kept pouring on gasoline until the remains sank down. Then they came with trucks and swept up the ashes. The whole town stank of burning flesh.”

Cochrane was a 24-year-old transport officer attached to a medical unit, and what he remembers most are the flies. “We had to take the poor buggers who were dying and lay them down outside,” he says. “I’d try and get less wounded men to sit with them and keep the flies off their faces so they could die in peace, but it was a waste of time. There were so many flies.”

And so many casualties, the aid station was struggling to cope. “We needed more ambulances to evacuate the wounded,” says Cochrane. “I’d spotted three trucks parked in a square, so I took a big bunch of keys and two volunteers to see if we could sneak in and get them started.”

They didn’t get far. They’d infiltrated less than 30 yards into enemy territory when a fascist defender lobbed a grenade at the patrol. It landed at Cochrane’s feet and he took the full force of the blast. Shrapnel shredded his back, arms and legs, and the detonator – a chunk of steel the size of a spark plug – punched a hole through his left thigh, slicing the femoral artery before exiting through his buttock. Cochrane’s companions were luckier. One, a fellow Dubliner called Paddy Blake, suffered a minor head wound; the other, an American volunteer, Tex, was unscathed.

“When I came to, there was a terrific amount of blood pouring out of me. It shocked the others, I think,” says Cochrane. “Tex said he’d go and find help, but that was the last time I saw him. We crawled into a house and I did what I could to patch up the holes, then sat on my leg to try and stop the bleeding.” I ask him if he feared for his life at this stage, and he nods. “Not because of the wound – I didn’t realise how serious it was and I reckoned I’d be all right if I could stop the bleeding – but because of the risk of being found by the other side before help came.”

As darkness cloaked the ruined town, the chatter, clatter, bark and boom of machineguns, rifles, anti-tank guns and artillery fell silent, allowing the screams of the wounded, and the pistol shots of those who found them, to echo through the streets. Promising to return with help, Blake slipped away, leaving Cochrane bleeding heavily in the cellar.

Until mid-July 1936, Belchite had been a prosperous country town of 3,516 inhabitants. The river flowed all year round, and the arbequina olive trees that surrounded the town loved the red earth. The olive oil for which Belchite was famous had brought wealth for some, and work for others, and the town supported two churches, a convent and a seminar Despite appearances, all was not well. Local landowners and businessmen were at odds with a socialist council trying to push through the land reforms proposed by Madrid. The olive-oil industry depended upon the vast numbers of unemployed who would work for bread or a few pesetas a day, but now the socialists were advocating workers’ rights, a minimum wage and the expropriation of the large estates.

The church was under fire, too: the clergy had been banned from teaching – inconveniently, since the church owned all the schools – religious festivals like Easter and Christmas had been replaced by International Women’s Day and Workers’ Day – and even funeral processions were prohibited. To one half of the population, Spain was on the threshold of a brave new world. To the other, it was teetering on the brink of the abyss.

Explore Books

• Book Extracts

• Book Reviews

• Books Group

• Audio Books

Times Recommends

• Superfreakonomics by Steven D Levitt and Stephen J Dubner
• The Times picks the 50 best paperbacks of 2009
• Vincent van Gogh’s letters show the soul of the artist