Today, as the country bends its collective mind to commemorate the great sacrifice in the First World War, I'll be thinking about three men who fought in the conflict and who came back changed for all time.

I met them a quarter of a century ago during a visit to the Western Front when it was still possible to see in them the young men who had gone off so hopefully to war. Each of them taught me a lasting lesson which reinforced General Sherman's remark to cadets bound for the US Army in 1879: "There is many a boy here today who looks on war as all glory, but, boys, it is all Hell."

The first veteran was a regular soldier who had made his career in the army and regarded it as both his job and his calling. Professional to his finger tips and blessed by an old soldier's insouciance and sense of the ridiculous, he took huge pride in being one of the last "Old Contemptibles", the name adopted by the British infantry of 1914 following a derogatory remark allegedly made by Kaiser Wilhelm II. He also took pride in the fact that as a young soldier he had been trained to fire his Short Magazine Lee-Enfield - the standard British infantry rifle of the day - at the legendary rate of "10 rounds rapid", putting up a withering field of fire so ferocious the attacking Germans thought it came from machine guns.

During his first encounter with the enemy during the retreat from Mons, the lines of German infantry got so close to his battalion's position that they stopped being a grey mass and he could see them as individual young men like himself. He survived the war and witnessed some dreadful incidents but he never forgot the sight of those young German soldiers lying in bloody heaps beneath the hot sun in September 1914.

The second veteran had served as a part-time Territorial soldier before the war and could not wait to get into action. His chance came early in 1915 on the Ypres Salient but his first taste of action was almost his last. While his battalion was waiting to go over the top a shell burst, spewing shrapnel over the trench and killing the man next to him, his best friend from his home village. A sergeant roughly shoved a sack into his hands and told him to pick up the body parts. Minutes later whistles blew down the line as his battalion went into the attack, creating fresh casualties: what had happened to the eviscerated friend was quickly forgotten.

But not quite. After the war he trained as a doctor and in old age took the chance to pay his respects at the headstone of his young friend who been killed all those years ago. "Look at him, he's still 19," he mused. "And look at me, I'm just a helpless old man."

The third veteran was the saddest. He had served as a volunteer, joining up quickly to do his bit in case it was all over before Christmas. The life and soul of any party, it seemed that nothing could ever knock him out of his good humour. In that sense he was not unlike the young man in Siegfried Sassoon's poem Suicide In The Trenches, which reads: "a simple soldier boy/Who grinned at life in empty joy."

After this veteran's death I learned that once a year he would lock himself in a room to hide the tears that came annually when he remembered his fallen colleagues. Some things are worse than death and some wounds are more horrific than others because they exist in the mind and cannot be seen. Today, I'll be remembering those three men and hoping that no others will have to experience what they went through 100 years ago.