Through the mud and olive trees, Scotsman Andy Crawford trudges over the Spanish fields where eight decades ago his grandfather William fought and died.

A Communist pipe-fitter from Glasgow, William was among tens of thousands of foreigners who fought in Spain's 1936-1939 civil war as part of the International Brigades.

Fearing the spread of fascism in Europe, they tried in vain to help Spain's Republican army fend off Francisco Franco's Nationalist uprising.

"There were no medals to be won, no wages to be earned and they were frowned on by half the world," said Andy, 66, standing on a hilltop near a stone monument to the brigades.

Along with 300 relatives and friends of the former "brigaders", Andy marched on February 21 with flags waving to mark the anniversary of the Battle of Jarama, named after a river southeast of Madrid.

In that three-week bloodbath in February, 1937, the International Brigades helped block Franco's drive to cut off the strategic road linking two Republican strongholds, Madrid and Valencia.

"People gave up everything just to come here and help," said Andy. "You've got to hope you can instil them principles into your own family."

- Historical memory -

Here and there among the olive trees stand crumbling stone enclosures -- machine gun emplacements used by Franco's forces. But there are no signposts and these spots are hard to find without a local guide.

"We would like the battlefields to be maintained, just as they are in other European countries and in the United States," said Daniel Loriente, a member of a local preservation society.

"We want them to be places of remembrance so that all this tremendous barbarity that happened in Spain never happens again."

Forty years since Franco's death, remembrance remains a raw topic in Spain.

Critics accuse the governing Popular Party (PP) -- a conservative movement originally founded by a former Francoist minister -- of neglecting the memory of Republican victims.

Things are slowly improving, however, Loriente said. A new regional heritage law in 2013 classified all the civil war trenches, bunkers and other fortifications around Madrid as sites of "patrimonial interest".

The regional PP government says it hopes to make more of them accessible to visitors.

"The law now protects all the civil war fortifications and that is a big step forward," said Luis de Fuente, a senior heritage official in the Madrid regional government.

- Digging up the bullets -

Near the Jarama battlefields in the village of Morata de Tajuna, Goyo Salcedo, 70, runs one of the few museums in Spain commemorating the Republican cause.

"There were problems to begin with, political obstacles," when the museum launched around 2006, Salcedo said.

"There were complaints to the police by people who said this was an arms depot. But there were others who said this has to go ahead because it is about historical memory."

In a barn behind a restaurant, his glass display cases are packed with relics of the brigades, dug up from the surrounding fields: bullet cartridges, artillery shells, knives and glass inkpots used by soldiers to write letters home.

The brigades brought together 35,000 volunteers from the United States, Ireland, Britain, France, Germany, Italy and beyond -- a fighting force unique in history, according to experts.

They fought with tanks, machine guns and hand-to-hand in the mud against Franco's Moroccan shock troops, who were backed by Nazi German warplanes.

The brigaders included intellectuals such as the British writer George Orwell, who survived being shot in the throat, and numerous working-class men like William Crawford.

About 9,000 of them were killed, estimates Paul Preston, a British historian at the London School of Economics and a leading expert on the Spanish Civil War.

"I don't think there's any comparable experience to that of the International Brigades," Preston said.

"They played a key role in most of the absolutely key battles."

Hundreds of International Brigaders died at Jarama but the carnage descended into stalemate until Franco won the war by finally seizing Madrid in 1939.

Historians estimate some 750,000 Spaniards died overall in the civil war.

- Tears of admiration -

At the Jarama commemoration, Patricio Azcarate, 95, gets shakily to his feet to stand with fist raised as the marchers sing the Internationale, the old Communist anthem.

Aged 18 and fresh from an international school, he served in 1938 as an interpreter helping Republican officers communicate with the International Brigades.

Now with a white beard and piercing gaze, Azcarate starts to cry when he remembers befriending a young US volunteer who days later was killed.

"I've no words to describe the admiration I have for the brigaders," he says.

"Not just the leaders in their command posts but also the soldiers who did the shooting and endured the bombardments. I'll always remember them."