As we continue the process of remembering and examining the events of a century ago, there is no shortage of significant events, or of significant deaths. But the killing of Field Marshal Sir Henry Wilson by two Irish Republicans outside his London home in June 1922, and the subsequent execution of his killers, certainly has a claim to stand out.

Partly it was due to the incongruity of two English-born Irish Republicans killing an Irish-born British imperialist.

Partly it was the rarity of the assassination of a sitting MP - the last had been Lord Frederick Cavendish in 1882, the next would be Airey Neave in 1979.

Partly it was the detail of the killing itself - the stricken Field Marshal dying on his doorstep, trying to unsheathe his dress sword so he could defend himself; his two assassins, both wounded war veterans - one with a limp, the other with a false leg - being pursued by a crowd of unarmed civilians and policemen, spectacularly failing to make their escape.

But mainly the assassination of Henry Wilson is remembered because it was the immediate cause of the Irish Civil War - and because of lingering questions over who ordered the killing.

In his meticulously researched and well-written book Great Hatred: The Assassination of Field Marshal Sir Henry Wilson MP, Irish Times journalist and historian Ronan McGreevy tells the story of Wilson, of his assassins, Reginald Dunne and Joseph O'Sullivan, and of the times that shaped them and brought them to their fatal encounter at the door of 36 Eaton Place.

Wilson was - and remains - something of a hate figure to nationalists, due to his intemperate public statements and visceral hatred of Irish independence.

Portrait of Field Marshal Sir Henry Wilson, by his childhood friend Sir William Orpen

Despite his later association with Ulster Unionism, he was himself from Longford, and his family were among the southern Unionists who opposed Partition, as they did not want to be left in an even more pronounced minority in a southern Ireland shorn of the north-east.

A committed British Imperialist, Wilson became a soldier (eventually - he failed the entrance exam for Sandhurst three times and for Woolwich twice), and served in various campaigns. A Burmese sword left him with a prominent scar on his face; Wilson enjoyed telling the story of how a letter addressed only to "the ugliest man in the Army" found its way to him.

He supported the formation of the UVF, but declined an invitation to become its chief of staff, offering instead to work behind the scenes to support the Ulster cause. He did so during the so called "Curragh Mutiny", intriguing in favour of British officers who stated they would refuse orders to march against Ulster Unionists.

This involvement increased the distrust politicians felt for Wilson. Prime Minister Herbert Asquith regarded him as a "poisonous tho’ clever ruffian", Lloyd George said there was "a streak of mischief - not to say malice - in his nature which often made trouble and sought to make trouble."

This dislike - more than reciprocated by Wilson, who detested most politicians - didn’t stop his eventual rise to the top job in the British Army, Chief of the Imperial General Staff, and the youngest Field Marshal since Wellington.

He rose to prominence not as a field commander, but as a planner (he prepared the British Expeditionary Force for deployment in 1914) and as an organiser (he instigated the joint Allied command in 1918, under his close friend Ferdinand Foch of France, which paved the way to final victory).

As CIGS, Wilson was sceptical about plans to recruit demobbed soldiers into the RIC - the Black and Tans. He complained that "there would be no hope of forming and disciplining this crowd of unknown men. It is truly a desperate and hopeless expedient bound to fail." In this he was certainly prescient, though his alternative was hardly likely to endear him to his fellow Irish: flooding the country with regular military forces in order to crush the IRA.

After his retirement, the Ulster Unionists found him a safe seat in North Down, and he was returned unopposed. The bile towards the Government previously confined to his diary was now unleashed in public.

He also became military adviser to the government of Sir James Craig. In private, he urged that Catholics should be encouraged to join the B Specials; in public, he appeared to be a partisan supporter of sectarianism.

And this was part of a pattern - he was wrongly held responsible for numerous outrages against nationalists. He had had nothing to do with the UVF’s pre-war gunrunning into Larne; he did not lobby for the execution of Kevin Barry; he was not responsible for the border skirmish between British and Irish forces at Pettigo in mid-1922.

Author Ronan McGreevy

But, as the author points out, "nobody was more to blame for the perception of Wilson in nationalist Ireland than Wilson himself." His public statements, including calls for a British reinvasion of the Free State, marked him out as a bitter and implacable enemy, which made him a tempting target.

Ironically, his killers were far less directly linked to Ireland than Wilson.

But Dunne and O’Sullivan, despite being English-born, identified as Irishmen, with Dunne describing the birthplace of himself and his parents as "the enemy country". While O’Sullivan came from a family steeped in Fenianism, Dunne’s Irish connections were more tenuous (his mother’s father came from Longford). He was converted to nationalism through an interest in Irish music and involvement in the Gaelic League.

Both had served in the British Army, O’Sullivan losing a leg and Dunne suffering severe leg injuries, but neither thought their subsequent IRA activities incongruous. As Dunne put it after their arrest: "The same principle for which we shed our blood on the battlefield of Europe led us to commit the act we are charged with."

Despite their dislike of Wilson, the British Government was outraged at his murder. It was immediately blamed on the anti-Treaty IRA, then occupying the Four Courts in Dublin, and pressure was put on Collins and his colleagues to take action. It was made clear that if they didn’t take action, the British forces remaining in Dublin would.

This was one of the main (though not the only) reason for the Provisional Government opening fire on the Four Courts garrison some days later, marking the start of the Civil War.

The problem is that the Four Courts IRA appeared to have nothing to do with it. There are, as the author outlines, four possibilities to the key question of who ordered the assassination.

He dismisses the idea that Dunne and O’Sullivan acted off their own bat; rules out the suggestion that the anti-Treaty side ordered it; and judges unlikely the (widely canvassed) suggestion that Collins ordered the assassination before the Truce and forgot to cancel it.

Which leaves the possibility that Collins ordered it after the Truce.

This may seem a surprising conclusion, but may make sense given Collins’s secret attempts to undermine Northern Ireland, and the general belief that Wilson was responsible for sectarian violence in Belfast.

Collins would also have expected the assassins to escape - nobody could have predicted that a one-legged man would have been one of the party - making the operation deniable.

Consider also that, after their arrest, Collins sent two of his top men, Liam Tobin and Tom Cullen, to London to try to rescue Dunne and O’Sullivan. The following year the same two men spontaneously gave Dunne’s parents the deeds to a house in Bray.

There is no clear proof one way or the other, but the author makes a compelling case on the basis of the evidence available.

This is an excellent book, which ranges much further than the story of Henry Wilson, Reginal Dunne and Joseph O’Sullivan, giving rich background detail on southern Unionism, the War of Independence, the establishment of Northern Ireland and the sectarian violence in Belfast which followed, as well as the Civil War. It deserves a wide readership.

Great Hatred: The Assassination of Field Marshal Sir Henry Wilson MP is published by Faber & Faber