Leaving a note simply saying "gone to Spain", a Merthyr Tydfil man left his family to go and fight General Franco and the rise of Fascism in 1937.
Timothy Harrington trekked through the Pyrenees, one of 4,000 British volunteers to join the cause.
Now, 80 years after he left wife Sally and five children, his grandson - actor Richard Harrington - has retraced his steps.
His diary forms the basis of the BBC documentary My Grandfather's War.
Such disappearances to join the International Brigade were not uncommon across the South Wales Valleys, where the plight of the democratically elected left-wing government in Spain resonated with the Labour movement of the mining and steel-working communities.
Research for the programme showed other instances, such as one woman waking up to find her husband had left, with a note saying "gone to the grocery shop".
The coup d'etat which carried General Franco to power in Spain and sparked the civil war arose because the land-owning class in Spain objected to the republican government's plan to redistribute farming land to the poor.
Unthinkable to the fascist parties and the Catholic Church, they launched an attack on the republican government. The war would claim more than 500,000 lives.
The British government was so keen to remain as far away from the war as possible, it made it illegal for anyone to travel to Spain and lend their services to the republicans.
They also sent warships out to sea to intercept any supplies which may have been on their way to the left-wing forces.
Yet miners of Wales risked their lives to travel to Spain and fight the spread of fascism.
"We are going on a jolly weekend to Paris," one of the Welsh men who travelled to Spain told a policeman, who questioned him and his friends as they left Britain via train.
"You better be back here on Monday or you will be in trouble," replied the policeman.
The men did go to Paris but, needless to say, they were not back by Monday.
Timothy Harrington and his 4,000 comrades who travelled to Paris were met there by communist agents, who would organise accommodation and protection from French spies.
They were transferred to Perpignan in the foothills of the Pyrenees where an old smugglers pass led them over the border without being detected by nationalist forces.
Once there, Timothy Harrington and his comrades joined the International Brigades, a group of about 30,000 foreign fighters from all over the world.
These men were given basic training in a town called Figueres. But thanks to the British-imposed embargo, equipment was basic and often out of date.
Harrington had been in Spain for four days before he was sent to Madrid to defend the city on 3 June 1937. The bullet holes from the battle can still be seen today.
Short of proper defensive support, books from the library were used to fend off the spray of bullets.
"Literature saved their lives," Richard Harrington says in the documentary.
After a 10 day battle, Timothy Harrington moved out of the city, as the republicans sought to cut off supplies to the fascists in the west by claiming the strategic villages of Brunete and Villanueva de la Cañada.
Brunete was taken by the republicans but Villanueva de la Canada was harder to conquer. The fascists held on for reinforcements and it arrived in the air.
Harrington was 35-years-old and only had half of his original lung capacity, when the Germans started dropping napalm on him and his fellow soldiers.
He collapsed with exhaustion and on 6 July 1937, his war was over. He went back to Merthyr Tydfil and his wife and family.
Timothy Harrington may have been back with his family, but the war in Spain raged on for another two years before Franco established full control.
By that point, more than half a million people had been killed and 100,000 more would be murdered in the years immediately after the war.
Children were taken from their parents and given to pro-Franco families. Many more were taken into the countryside and shot dead.
To this day there are still 150,000 people who have not been found.
What memory does Spain hold of its fascist past and the civil war? Well, there are graves to the fallen soldier, but only on one side, and a huge shrine to General Franco made completely out of grey stone, which Richard Harrington visits towards the end of the documentary.
While he is there, he meets modern-day Franco supporters paying homage to their icon but there are also school children there, ignorant to the level of the atrocities committed in their grandparents' lifetime.
The stark contrast has a striking impact on Richard Harrington.
"This is not the sunny Spain you see when you fly over here. This is quite sick. There's no love here at all. Is that how you portray a Christ as colourful as he was? In grey? Those children over there have no idea where they are coming. Laughter is the sound that should be here," he said.
As he meets the children, they do laugh. Their teacher asks them what is better - democracy or dictatorship?
"Democracia!" they answer, the word echoing around the shrine of a brutal dictator.
Timothy Harrington may not have finished his war, nor did his comrades stop fascism from triumphing and ruling for 36 years. But eventually, fascism was defeated in Europe.
The role of the young men from Wales and the rest of the United Kingdom who left their families to contribute to this effort deserves to be remembered.
- Watch Richard Harrington: My Grandfathers War on BBC Two on Saturday, 15 April, at 21:00 BST or on the BBC iPlayer