It's not just ordinary people whose ancestors fought in the First World War.

It's a shared chapter in the genealogies of U.S. President Barack Obama and Russian President Vladimir Putin. For British Prime Minister David Cameron, it evokes proud family memories of valour tempered by sorrow. For German Chancellor Angela Merkel, it involves a grandparent whose actions are tinged with mystery.

On Thursday, in the Belgian city of Ypres near the once-blood drenched battlefields of Flanders, the leaders of the European Union's 28 nations will mark the 100th anniversary of the beginning of the conflict that wreaked death and destruction across their continent.

For some of the men and women who will be present, the Great War is not the stuff of distant history. It is the story of their own families, often just two generations removed.

David Cameron

More than 65 million men were mobilized to fight in the First World War, including the grandfathers and relatives of many of today's heads of state and government.

Three members of Cameron's extended family were killed in action, one near the spot of Thursday's ceremonies.

In March, the British prime minister spoke with pride about his great-great uncle, Capt. John Geddes, who served in the Canadian Army and was killed at the Battle of Kitcheners' Wood, part of the 2nd Battle of Ypres — the first time during the war that the Germans used poison gas.

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Two visitors walk at the St. Julien Canadian First World War memorial in Langemark-Poelkapelle, Belgium. (Geert Vande Wijngaert/Associated Press)

"On the 22 April 1915, the Germans launched their first poison gas attack on the Western Front, leaving a gap in the line four miles wide," Cameron said.

Two Canadian battalions were ordered to fill the gap, Cameron said, "which they did, without prior reconnaissance, breaking through any obstacles with rifle butts, into a wall of German machine gun fire."

"My relative was the one who helped lead the way," he said. "I am proud to be linked, in some small way, to this extraordinary heroism."

Eighty per cent of the Canadian troops were killed or wounded in the assault.

According to a death notice in the Toronto Star, Geddes was born in Chicago — the son of a Scottish grain merchant — and lived in Winnipeg for 12 years. He was 37 when he died. His name is on the Menin Gate memorial in Ypres, where Cameron and the other EU leaders will hear the Last Post performed and dedicate a commemorative peace bench.

Geddes was not the only member of Cameron's extended family to die in action. John Geddes' brother, Lt. Alastair Geddes of the Royal Scots Fusiliers, was killed two months later at the age of 23.

Another of Cameron's great-great uncles, Capt. Francis Mount of the Royal Berkshire Regiment, died in October 1915 at the Battle of Loos — the first battle in which the British used poison gas.

Angela Merkel

A photograph that recently surfaced of one of Merkel's grandfathers has ignited speculation he might have fought against Germany, alongside fellow Poles and Polish emigres to America serving on the Western Front. But there's been no hard proof.

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A photograph that recently surfaced of one of German Chancellor Angela Merkel's grandfathers has ignited speculation he might have fought against Germany. (Fabrizio Bensch/Reuters)

Merkel was born Angela Kasner, the Germanized name that her paternal grandfather, Ludwig Kazmierczak, is said by one of her biographers to have adopted in 1930. Reporters from a German all-news TV station found documents in Poznan, western Poland, saying that Kazmierczak was drafted in 1915 at age 19 into the Prussian army. Poznan belonged to Prussia then.

The photo, from after the war in around 1919 or 1920, pictured Merkel's grandfather in the uniform of the so-called Haller Army, a Polish force used by the French army against Germany.

The photo has led the Polish press to speculate that Kazmierczak might have ended up in French hands as a prisoner or after deserting from the Prussian army, then joined the Haller Army and fought against Germany. But there is no conclusive evidence.

Barack Obama

The First World War was not only fought in the trenches of France and Belgium; battles raged in Eastern Europe, Turkey, the Middle East and Africa as well.

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The First World War affected U.S. President Barack Obama's paternal grandfather. (Larry Downing/Reuters)

In East Africa, Obama's paternal grandfather was apparently one of those whose lives were transformed as a result. In his autobiography Dreams of My Father, Obama includes a lengthy description, attributed to his grandmother, of his grandfather's life.

The only reference to the First World War in the book is that Obama's grandfather "made himself useful to the white man and during the war he was put in charge of road crews."

However, another written history of the American president's family says Hussein Onyango Obama enlisted in the King's African Rifles Carrier Corps, which fought against the Germans in East Africa. Casualties were "astonishingly high," Pete Firstbrook notes in The Obamas: The Untold Story of an African Family.

The experience seems to have marked Onyango for life.

"Of 165,000 African porters, more than 50,000 died, a death rate higher than the average on Europe's bloody Western Front," Firstbrook wrote. "When he [Onyango] returned, he declined to reside in his father's compound, but settled instead into an army-issue tent."

"People thought he was crazy," Firstbrook wrote.

Onyango's personal style, too, was different from the rest of the family. Unlike them, he ate at a wooden table with a knife and fork, wore European clothes and was obsessive about cleanliness. He admired the British, Firstbrook says, "especially their discipline and organization," and by the mid-1920s, was making a good living as a cook for British families around Nairobi.

Vladimir Putin

Before the war, Putin's paternal grandfather, Spiridon Putin, was already working as a chef. The online National Encyclopedia Service of Russia says he was "called to the front," though it is not clear in what capacity. As a professional cook, he may have been put in charge of a mess hall rather than being a fighting man in the czarist army.

The Russian leader's maternal grandfather, Ivan Shelomov, born in 1904, was too young to fight in the war.

Francois Hollande

Both grandfathers of French President Francois Hollande were mobilized. Gustave Hollande, on his father's side, became an officer. Maternal grandfather Robert Tribert, a tailor by trade, became a corporal, assigned to the Army Theater.

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Both grandfathers of French President Francois Hollande were mobilized in the First World War. (Philippe Wojazer/Reuters)

"Practically all our politicians have ancestors who fought in the war because three-quarters of the Frenchmen of that generation took part," Jean-Louis Beaucarnot, author of the book Our Families in the Great War, told The Associated Press.

In a war that claimed more than one million French soldiers and left another four million injured, both of Hollande's grandfathers made it through alive and unharmed, Beaucarnot said.

Giorgio Napolitano

The First World War's memory feels especially close to Italy's 88-year-old President Giorgio Napolitano, who has called the conflict "the first great collective experience for the Italian people." His father Giovanni was a reserve officer who fought at the front lines during the Battle of Asiago in the foothills of the Alps in 1916, a surprise counteroffensive launched by the Austrians. He later wrote a book about his experiences, writing that the horrors of warfare led to a vital unifying moment for the country.

"One suffers immensely, but returns bettered," Napolitano's father wrote. "Everyone suffers who makes war, everyone sacrifices, but the survivors have inherited a new sense of life."