War is ambivalent; for the powers that decide to wage it, it is as ancient as man, even older than Homer and is caused by expansionist greed and domination if invariably justified by moral righteousness, usually that of the perpetrator. Various interested parties may also be relied upon to form self-serving alliances of convenience. No wonder the gods regard war as sport.

But for the men who do the fighting, war combines many things including lofty, often confused, notions of idealism and bravery motivated by family honour, love of country and the glory of an adventure defying death. The reality is very different; it is about fear and injustice compounded by suffering, squalor, madness and the unrelenting guilt endured by survivors.

Soldiers fall as expected but so too do the innocent – children on their way to school, a girl cycling to fetch eggs is killed by a stray bullet, or boys, by an unexploded landmine while playing football. War is random, some live, some die, many suffer and everyone remembers.

Writers continue to be drawn to it as the only literary theme more powerful than romantic love and far more interesting, more real. Above all, most writers are aware that ambivalence is the secret source of stories and nothing is more ambivalent than war.

That essential element, ambiguity, encapsulates Ireland’s involvement in the Great War: 200,000 Irish men fought in the army of the old oppressor. Generations have referred to the legacy of the first doomed Anglo-Irish Treaty and the Civil War it caused, but many of the tensions go further back to the Great War. What worse act of betrayal could there be than to have served in the British army?

The Anglo-Irish regarded it as serving one's king as well as appeasing one's relatives. For a working class man, though, be he from the city or the depths of the countryside, it was a job and a monarch's shilling was much the same as that of any employer. There was an additional element: as Jerry Crowe, the Wicklow village boy with a feel for horses in Jennifer Johnston's How Many Miles to Babylon? (1974) reasons: " . . . the Germans are going to fix all those eejits in Europe, the British are going to fix the Germans, and we . . . We are going to fix the British."

Social class

There were those who may have seen a stint in the ranks of a professional army, even one belonging to the traditional enemy, as a useful training ground for rebellion. There would be those who would criticise the Easter Rising for being opportunistic and not quite cricket. After all, the enemy was elsewhere engaged. Home Rule was a carrot. The Ulster Volunteers were established to threaten the hopes of Home Rule and a bewildered John Redmond could be accused of well-intentioned emotional blackmail. No wonder the relevance of the Allied cause is frequently reduced to a mere footnote.

Johnston's novel, long-established as a curriculum text in Irish schools was published eight years before Michael Morpurgo's War Horse. The pairing is not as odd as it may seem. Both books confront the notion of social class and both are about love. Johnston's Alec, son of the Big House, has only had one friend in his life, Jerry the aspiring nationalist. Jerry disobeys an order to go find his father, a career soldier in the British army, and Alec instead of leading the firing squad, shoots his friend and in turn secures his own execution.

In War Horse, the message is much the same, albeit apparently more simplistic. Devastated when his beloved horse, Joey, is commandeered by the army, and assigned to an officer, Albert enlists and pursues Joey to France, intent on saving him.

Both books were part of a thematic revival in Great War literature. No, war never went away. “Don’t forget” remarked Johnston once to me in an interview: “When I was young there were still people who had fought in it. People who had lost brothers, fathers, uncles, sons. They were all around me.” Calmly and deliberately Johnston a novelist with a feel for social and cultural nuance has always responded to the several faces of Ireland and Irishness.

Above all, war is the story that best explains the ambivalence of loyalty, belief, conscience, the essential business of being human. Humanity costs Alec his life. Morpurgo would echo Johnston's theme in another war novel, Private Peaceful (2003), in which two brothers go to war and when the older one attempts to help his younger brother he too is seen as having defied army rules and is executed. Military procedure, it must be conceded, does not fare well in the more superior of literary war novels.

Horrors of the Somme

Writers including poets such as Wilfred Owen, Rupert Brooke, Francis Ledwidge and Edward Thomas were immortalised not only by the works but by their individual mythologies, while Erich Maria Remarque's All Quiet On the Western Front (1929) gave an honest account of what it is was like to be very young, if suddenly old, disillusioned and fighting in a war he didn't believe in, for an army staffed by superiors he could not respect. True, he was on a side that not only lost, it was disgraced.

Yet consider JRR Tolkien, rarely acknowledged as a member of the Lost Generation of which he undoubtedly was, having experienced the horrors of the Somme, which certainly influenced The Lord of the Rings: he fought for the victors – and remained quietly traumatised throughout his long life.

Virginia Woolf did not go to war, yet for all her remoteness she was as marked by it as any of her generation. The menace of the Great War and the shadow it casts upon the lives of the Ramsay family undercut her masterpiece To the Lighthouse (1927). In Mrs Dalloway (1925), the musings of the central character, a society woman going about her day preparing for that evening's party, are interspersed with the dilemma of Septimus Warren Smith, a traumatised war veteran haunted by the death of his friend Evans at the front. Faced with involuntary commitment to a psychiatric hospital, he leaps from a window and dies.

It is interesting to concede that the initial literature of the second World War as represented by, say Norman Mailer's edgy and quite cryptic debut novel The Naked and the Dead (1949), is strongly autobiographical and has little characterisation and even less compassion.

The literature of the second World War rarely achieves the lyric intensity and pathos of that of the Great War. It was not until the publication of JGBallard's detached if powerfully evocative Empire of the Sun (1984) that British literature, so rich in Great War writing, could claim its defining second World War novel – allowing for the fact that Evelyn Waugh's Sword of Honour trilogy, Men at Arms (1952), Officers and Gentlemen (1955) and Unconditional Surrender (1961), may be the finest writing but the Waugh the stylist is not the most sympathetic of writers.

Rampaging nationalism

Ironically and entirely by coincidence within months of the publication of Ballard's novel – so different from his other work – the Irish playwright Frank McGuinness was to probe hidden emotions in audiences all over the world, not only in Ireland, with his third play Observe the Sons of Ulster Marching Towards the Somme, first performed in the Peacock Theatre in Dublin, in February 1985. In that landmark work, a group of young Ulster volunteers fighting on the Western Front see the parallels with the Battle of the Boyne.

In the closing scene, Kenneth Pyper, the only survivor, intones with a near-Biblical rhetoric: “God in heaven, if you hear the words of man, I speak to you this day. I do it now to ask we be spared. I do it to ask for strength. Strength for these men around me, strength for myself. If you are a just and merciful God, show your mercy this day. Save us. Save our country. Destroy our enemies at home and on this field of battle. Let this day at the Somme be as glorious in the memory of Ulster as that day at the Boyne, when you scattered our enemies. Lead us back from this exile . . . Let us win gloriously . . .”

Ulster is seen as a nation in its own right in the midst of a global conflict caused by the rampaging nationalism which shattered the Austro-Hungarian empire.

By then of course novelists everywhere were (and continue) mining the apparently inexhaustible seam of the Holocaust as the most dominant aspect of the second World War upon which to base novels. The systematic mass genocide of the Jewish people across Europe remains humanity’s most vile crime; it is the biggest story in literature as well as history. Many of the Holocaust novels are important, not all of them are great, or even that good. The literature of the Great War is somehow closer to art; it has certainly inspired beautiful writing and continues to do so, particularly since about 1980.

Within months of Margaret Thatcher commencing a reign that would not only destroy a traditional way of life, that of the miners, and later return Britain to war, in the Falklands, a short novel was published which suggested that British writers continued to be haunted by the Great War. JL Carr's A Month in the Country told the story of two soldiers who had returned from the Front, and both were attempting to makes sense of their lives.

One of them, the narrator, was obviously traumatised and had been left with a speech impediment and nightmares. He does have a project: the restoration of the wall paintings in an old rural village church. The other man is digging for a missing grave. Both men are lost.

Carr’s novel was shortlisted for the then Booker Prize. Yet again it outlined that the most damaged casualties of war are the lost generation, the veterans. Carr’s novel is unusual in that it looks at the aftermath; much of the Great War literature describes what it was like, the sheer immediate hell of it. The vital element in creating powerful war literature is personal, as well as ascending levels of craft and art. In the absence of direct experience, writers turned to source material and some make better use of that than others.

Unbroken silence

British writers Robert Graves and the poet Siegfried Sassoon both had served. Graves described his two years at the Front, during which he was seriously wounded, in his urbane if barbed – and initially, heavily censored – autobiographical account, Good-bye to All That (1929). But a memoir that laid it all uncomfortably bare, as far as the authorities were concerned, and would also prove a vital influence on a subsequent war trilogy by a British writer who made effective use of the wealth of first-hand accounts, is Sassoon's thinly fictionalised autobiographical trilogy: Memoirs of a Fox-Hunting Man (1928), Memoirs of an Infantry Officer (1930) and Sherston's Progress (1936).

The second volume is the best. Sassoon's jaunty, disarming voice subtly registers his change in attitude as war becomes less a jolly adventure taking place in the mud of Flanders and more something which revolts him. He makes his protest clear. Reflective rather than introspective, he is an observer conscious of the changing sky, the unbroken silence, the search for an enemy he can't even see.

He is not as soulful as Remarque's Paul Baumer; a country lad given to writing poetry which one can bet is romantic. Sassoon was a satiric poet; there is always an edge. Early in the narrative he says he is in the middle of reading Thomas Hardy and recalls thinking: "I didn't want to die – not before I'd finished reading The Return of the Native." Baumer does not crack jokes.

One senses Sassoon’s awareness of watching history unfolding with every weary gesture, each sudden death: “Soon they [his comrades] had dispersed and settled down on the hillside, and were asleep in the daylight which made everything seem ordinary. Nonetheless I had seen something that night which overawed me. It was all in the day’s work – an exhausted Division returning from the Somme Offensive – but for me it was as though I had watched an army of ghosts.”

No one who had read her previous novels, serious, political and compelling as they were, would have predicted that Pat Barker, admittedly a former history teacher, would have written a multi-layered Great War trilogy quite as brilliant as Regeneration (1991), The Eye in the Door (1993) and the final volume, The Ghost Road, which won her the 1995 Booker.

Prolonging the war

Drawing on diaries and letters written at the Front by a range of soldiers from all social backgrounds, she brought the horror to life. She also consulted and assessed war office material. Most important, she conveyed the rage and frustration. Barker made extensive use of Sassoon’s wartime experiences and his treatment under the legendary neurologist WHR Rivers in Craiglockhart in Scotland.

When Sassoon turned against the war, he did so most emphatically and presented a document, A Soldier's Declaration, in which he accused the war office of deliberately prolonging the war. Barker made it clear that the ordinary soldiers were treated as cannon fodder. The injured, many of whom had experienced gas poisoning were brought back to Britain, patched up and returned to the Front.

There is no doubt that Barker wrote a very different war novel – there was nothing romantic.

The second volume, The Eye on the Door, the finest, and most critical, was cautiously received. A bold, brave book, it said a great deal about Britain in 1918 in the aftermath of a not so glorious victory. It took on social class and the decisions that were made. Published in the same year, 1993, as Sebastian Faulks's acclaimed, somewhat conventional Birdsong, it is the better novel. Of all the praise which has been deservedly lavished on The Regeneration trilogy, Barker's supreme achievement was her exploration into the minds of men damaged by war.

Russell Crowe's important new movie The Water Diviner inspired not by a novel but by one poignant real-life sentence has directed attention to the Dardanelles and the Gallipoli campaign in which Australian and New Zealand troops were killed, as were an estimated 5,000 Irish soldiers serving in two regiments. Yet the Western Front continues to preoccupy novelists.

Sebastian Barry touched on the conflict faced by an Edwardian Irishman serving the British crown in The Steward of Christendom (1995). A Long Long Way (2005) continues Barry's theme of stories based on his family. In this novel, Willie Dunne sets off to the Front with the Royal Dublin Fusiliers. Barry places the entire historical debate on the table and eloquent exasperation does the rest: "'I don't understand this volunteer thing,' said Willie. 'You're volunteers, you say – but, you know, I'm a volunteer too – I volunteered for the army.'

‘Ah Jesus, Willie. That’s different altogether. You’re a volunteer for fucking Kitchener. You can’t be this thick. Look it, boy. The Ulster Volunteers were set up by Carson to resist Home Rule. So then the Irish Volunteers were set up to resist them, if necessary. Then the war came, as you may have noticed, and most of the Irish Volunteers did as Redmond said and came into the war, because Home Rule was as good as got. But a few broke away and that’s who you saw on the lovely streets of Dublin! Of course, Willie, the Ulster Volunteers came in too, but not for Home Rule, for God’s sake. But for King and country and everything kept as it is. You see now?’”

Except nothing was kept as it was; Europe was redrawn; empires fell. The rumour of an assassination involving the emperor's nephew that some of the bored officers in Joseph Roth's The Radetzky March (1932) had doubted turned out to be very true.

We now live in an age of suicide bombers and, increasingly, computerised drone attacks. War is strangely impersonal. The only human aspect is death.

It is not that surprising that writers continue to be drawn to the Great War; it was the end of war as a theatrical pageant and the beginning of an even more terrifying reality: mechanised war which in turn has been supplanted by digital technology.