Five men who survived the carnage, the youngest is 100, visited the British cemetery at Arras and the battlefields where the cream of an entire generation was wiped out on 1 July 1916.
At 7.30 that morning after the German lines had been pounded for a week with one and a half million shells, more than 420,000 troops, most of them British, went over the top to launch the bloodiest battle in military history.
The Allies vastly outnumbered the Germans yet by nightfall the British alone had suffered 57,470 dead and wounded. The Germans lost just 185.
Most of the dead were volunteers who answered Kitchener's call to arms fight for their country and were sent into battle with a bare minimum of training.
Donald Hodge, 101, from East Storrington, west Sussex, said: "The last thing I thought about before the war was joining the army, the old regulars were a bunch of scallywags. But when the call came, it seemed the right thing to do.
"We had no uniforms or any equipment for months, and we lived in tents because there were no huts to put us up in.
"My memories of the Somme are very mixed. Everything was haywire. We were playing it by ear. The German Chiefs of Staff were cleverer than ours and they had deep dug-outs so most of our bombardment had no effect.
"The first week of July was a wholesale slaughter, no end of my friends died. The bodies were piled up high.
"We were young and fit and we took it all in our stride. Whatever the orders were, we just obeyed them, that's how we were then."
Mr Hodge, who was in the Royal West Kent Regiment, added: "I feel it is a duty to come back. I must come back for the sake of my friends who lie here. It could so easily have been me instead of them.
"We were very, very close, we trained together, we marched together, we fought together, we were closer than brothers but you learned to lose friends without unduly grieving, otherwise we would have gone mad."
Mike Lally, 102, from Salford, Greater Manchester was visiting his older brother James' grave yesterday for the first time. "That's the reason I came over this time, to find my brother's grave," he said.
"Our mother used to visit it every year but she never would tell us where it was. She never used to talk about it."
The First World War Veterans' Association, who brought Mr Lally and 12 other veterans back to France, traced the grave through the War Graves Commission and Mr Lally held onto his son's arm and broke down in tears as he stood before the simple white headstone.
Afterwards he said: "All them years I had never seen it." Looking around at the rows of neat white gravestones in the British cemetery near Arras on the Somme, he added: "All them lives. What a waste. It was the biggest mistake ever. They threw so many men into that battle, it was a complete waste."
Norman Booth, 100, from Golcar, near Huddersfield a 1914 volunteer with the Duke of Wellington's Regiment, said: "They're shocking, my memories of the Somme. I thought it was a disgrace that they should lose all those men in one day."
Mr Booth, awarded the DSM and Bar, added: "To see all those lads slaughtered in that swamp, it broke my heart. It made me very angry. It still does."
The years have clearly done nothing to assuage the anger felt by many of the veterans who saw so many of their contemporaries perish. Tom Brennan, 100, a gunner, who now lives in a Liverpool nursing home, said of the first day of the Somme: "I went up there that evening and you could hardly see the ground for the dead."Reuse content