Soon after noon on July 20 1944, a bomb exploded in the Wolf's Lair, Adolf Hitler's military headquarters in East Prussia. The intended target was the Fuhrer.
If the plan to kill Hitler and launch a coup had been successful, Carl Goerdeler would have taken over as leader of the new Germany.
Colonel Claus Von Stauffenberg, a dashing war hero who had placed the bomb in an attaché case in Hitler's briefing room, was to be Minister for War.
And my grandfather, Fritz-Dietlof Von Der Schulenburg, was set to become Minister of the Interior.
As the bomb ripped through the barracks shed, killing four people and wounding others, Fritz was ready and waiting in Berlin for the seizure of power. It was my grandmother's birthday on that day, but he could not celebrate it with her.
As the historian Ian Kershaw put it: "What could go wrong for the plotters on that day did go wrong.'' Stauffenberg was forced to hurry when he was preparing his bomb in a side room, because he was called away to a phone call.
There was no time to set a second explosive, which would have doubled the impact. Also, a heavy oak table leg helped to shield Hitler from the blast.
Hitler staggered out of the hut with nothing worse than splinters, minor cuts, and burst eardrums.
In the grand scheme of things, the failure of the coup meant that the war continued for another 10 months. On a personal level, Valkyrie shaped the destiny of all four of my grandparents, and brought about the eventual meeting of my parents in Ireland.
Both Fritz and my other grandfather, Peter Bielenberg, were arrested.
Fritz, a Prussian count, had been a relentless plotter against Hitler, involved in several assassination plots before Valkyrie.
As vice-president of the Berlin police in 1938, he planned to have the gates opened to the chancellery in Berlin so that a commando unit of resistance fighters could seize Hitler. This plot was shelved along with many others.
In 1940, Fritz hoped to have Hitler shot during a victory parade through Paris in late July. Sharp-shooters were to be placed in the crowd. But the elusive Nazi leader never turned up.
Early on in his career, Fritz had been an enthusiastic Nazi. He hoped the National Socialists would restore the pride of a country humiliated in the First World War. But he was soon disillusioned by the corruption of the new regime.
He became a key intermediary in the Resistance, helping to bring together a disparate group of army commanders, conservative intellectuals and socialists.
Fritz and Stauffenberg became friends in the 1930s; they shared an aristocratic background, and a certain reckless streak. But before the outbreak of war, Stauffenberg -- the man who eventually came closest to killing Hitler -- was somewhat wary of my grandfather's plans for a coup.
It was only in 1943, after Stauffenberg was badly wounded on the North African front -- losing his right hand, two fingers on his left, as well as his left eye -- that he emerged as the driving force of the Resistance. Stauffenberg sent an electric charge through the secret opposition.
Fritz and Stauffenberg spent their last Easter together at my great aunt's country home at Trebbow in north Germany. Fritz's family, including my mother Charlotte, then only four, had been evacuated there to escape the air raids on Berlin.
During that Easter, the family told ghost stories. The men discussed Shakespeare, and their plans for the future. Stauffenberg's driver, a magician in civilian life, performed magic tricks for the children.
My great aunt Tisa recalled how Stauffenberg "insisted on lighting his cigarette by himself, pressing the matchbox with the stump of his missing arm".
This dexterity was crucial when he was preparing the bomb intended for Hitler a few months later.
After the explosion in the Wolf's Lair in July, Stauffenberg was convinced that Hitler was dead, having seen a body stretchered away from the scene covered in Hitler's cloak.
He sped away in a car, before catching a flight to Berlin.
When he arrived at army headquarters in Berlin, Fritz and the other ringleaders were waiting. Army chiefs had already sent out orders to arrest key Nazi officials and mobilise army units.
Nazi party offices were to be occupied; but with Hitler still alive, the mobilisation had only partial success.
Confusion reigned as different army commanders issued different orders, but gradually Hitler's forces regained control.
Stauffenberg was arrested and summarily shot, shouting to his executioners: "Long live sacred Germany!''
My grandfather was held captive in Stauffenberg's office until the prominent SS man Otto Skorzeny arrived to help to quell the uprising. Skorzeny (who was later made welcome in Ireland) approached Fritz and the other prisoners one by one, tearing off their medals and throwing the decorations into a steel helmet on the floor.
Fritz was convicted at a show trail in the People's Court. To the fury of the judge, he said: "We have accepted the necessity to do our deed in order to save Germany. I expect to be hanged for this and I hope that someone else, in luckier circumstances, will succeed.'' Fritz was duly hanged from a meat hook in Plotzensee prison on August 10.
My mother's family were kept under house arrest in their home in the countryside.
My grandmother and her six children later fled the advancing Russian army on a horse and cart.
The events of July 20 also threw the lives of my other grandparents, Peter and Christabel Bielenberg, into turmoil. Although Peter was not directly involved in the plot, he had been active in the Resistance a few years earlier and was a close friend of one of Valkyrie's ringleaders, Adam von Trott. Christabel, an Anglo-Irish woman, had married Peter in 1934 after coming to Germany to study singing.
Just days before the July assassination attempt, Trott sent Peter a message urging him to come to Berlin immediately. When the plot failed, Peter had a plan to spring Adam from captivity, by ambushing the convoy that took him from prison to Gestapo headquarters for interrogation.
He went to the factory where he worked to fetch a gun from an armoury, but the Gestapo were waiting for him.
Peter was imprisoned at Ravensbruck Concentration Camp. When Christabel went to meet Peter in the camp, they clasped hands and Peter managed to plant in her palm a matchbox that carried a message outlining the story he had told his interrogators. Alibis were clearly set out. With typical effrontery, she later turned up at Gestapo headquarters, asking to be interrogated. She bluffed her way through the interview, using Peter's concocted stories. Peter was eventually released to an army punishment camp, but slipped away and hid in the Black Forest.
Peter and Christabel had long had a hankering to move to Ireland, and bought a farm near Tullow after the war was over. Having survived the war, with three young sons (including my father Nick), Christabel and Peter were keen to help the families of the German Resistance who were not so fortunate. Christabel helped to set up a fund for the families.
They invited the children of the executed plotters to Ireland for holidays. The daughters of Adam Von Trott came. Other children of executed leaders, by then teenagers, were also frequent visitors.
Among them was Charlotte, daughter of Fritz. That is how my parents met. Charlotte von Der Schulenburg and Nick Bielenberg married in 1958. They were two young people, brought together in County Wicklow by a plot that narrowly failed to kill a tyrant.
Tom Cruise's battle to make the Stauffenberg movie 'Valkyrie:' Page 21