One hundred years ago this week, political debate in this country was fixated on the prospect of conflict. It was not war with Germany that alarmed our parliamentarians, though; it was civil strife across the Irish Sea.

On April 7 1914, the House of Commons passed by a majority of 80 the Government of Ireland Bill — or the Home Rule Bill, as it was more popularly known. It was the third time that the legislation had been before the House, having twice before been rejected in the Lords, and it was intended to be the last.

Following the passage of the Parliament Act in 1911, the lower house now had the power to force it on to the Statute Book by overriding the objections of the peers. But the consequences of doing so risked calamity. The unionists of the north of Ireland were mobilising to resist Home Rule and the spectre of civil war hung heavy over the land.

Yet the Liberal-led government of Herbert Asquith, needing the support of Irish nationalists, was determined to push on. In July, King George V convened a conference between the two sides, led by Sir Edward Carson for the unionist covenantors and John Redmond for the nationalists, but failed to achieve a compromise.

Two weeks later, the country was at war with Germany and the Bill sank. Had the Great War not intervened and Home Rule had gone ahead — with special arrangements for Ulster, short of partition — what would have happened?

It is one of the great “what ifs” of modern British history. Some historians today believe Redmond could have delivered a constitutional and largely peaceful settlement. Home Rule could have been achieved without the militant separatism that characterised the Easter Rising in 1916 and subsequent conflicts from the civil war of the 1920s to the Troubles.

Only now, a century later, are we finally ending the schism that opened then with the collapse of the only chance that existed of retaining a semi-autonomous Ireland within the United Kingdom. The state visit of Michael D Higgins, the president of Ireland, is the last act in a 200-year clash between the tradition of armed struggle against the British state and those who, like Redmond, favoured a constitutional settlement.

And to that end, it is not the presence of President Higgins in London that is the most remarkable aspect of this momentous event, but the attendance at a banquet at Windsor Castle on Tuesday of Martin McGuinness, the deputy first minister of Northern Ireland. To see the former IRA commander in his white tie seated at a table with the Queen is an affront to many victims of the terror group that he led for so long. Yet it is also an extraordinary demonstration of how far armed-force republicans have moved from their absolutist demands for a united Ireland.

McGuinness is, after all, an elected politician sitting in an assembly of a province that remains within the United Kingdom. He has already shaken hands with the Queen; now he is a guest in her home. Anyone suggesting such a prospect just 20 years ago would have been considered certifiably insane. For Mr McGuinness is no ordinary Irish republican. He was arguably the movement’s most significant figure throughout the Troubles, the very personification of the “armed struggle”.

The author Kevin Toolis, in his book Rebel Hearts, said of McGuinness in 1995: “No other living person is a greater threat to the British State.”

McGuinness joined the Official IRA as a young man in 1970, but the “Stickies”, as they were known, were considered too unwilling to confront the British Army and Loyalist gunmen, so the Provisionals were formed. It is believed that he twice acted as chief of staff and allegedly commanded the IRA in his native Londonderry during a ruthless campaign of bombings and assassinations.

At the height of the Troubles in the 1970s, hundreds of people were being killed every year and bomb explosions and street riots were a daily occurrence. We know how important a figure McGuinness was because in 1972, at the age of just 22, he was flown secretly from Ulster, with Gerry Adams and other imprisoned IRA leaders, to negotiate with William Whitelaw in a failed attempt to end the bombing campaign.

The IRA waged a campaign on the British mainland that targeted police officers and soldiers as well as civilians. Huge damage was caused to infrastructure and the economy of the City of London by a series of massive bombs in the mid-1990s. But while governments and politicians may talk blithely of new beginnings, the relatives of those killed do not so easily forget the carnage and the grief the IRA caused.

Victor Barker, whose 12-year-old son James was killed by the 1998 Omagh bomb, travelled to Windsor on Tuesday to stage a one-man protest against McGuiness’s presence at the banquet.

“A terrorist in white tie and tails is still a terrorist,” he said.

Doubtless, there are many fathers, mothers and siblings who share Barker’s feelings. Concession and rapprochement also bring with them the danger of moral equivalence, where the actions of British soldiers sent to keep the peace in the province are judged to be on the same ethical plane as putting a bomb in a city centre waste paper bin to explode in the midst of crowds of shoppers.

Yesterday, Francie Molloy, a senior Sinn Fein politician, said the time had come to draw a line under all past activities by acknowledging an amnesty for terrorists and members of the Armed Forces alike. Despite the longest judicial inquiry in British history into the events of Bloody Sunday in 1972, followed by an apology by David Cameron on behalf of the British state, members of the Parachute Regiment are still subject to a police investigation that could result in prosecutions.

“We have to find a way of dealing with the past which is not all about vengeance, and that goes for all sides,” said Molloy.

Should the Government take him up on this offer? Is it even his to make? The whole “peace process” was, from its inception, a series of compromises of varying degrees of shabbiness. In 1997, Sinn Fein was allowed to take part in talks for the first time by the incoming Labour government, even though the IRA — which had called a ceasefire the previous year — still refused to give up its weapons. Disarmament — or decommissioning. as it was known in the peculiarly euphemistic language of the process — would become a bargaining chip to be played in the negotiations as they unfolded.

In the end, the weapons were not so much destroyed as verifiably “put beyond use”. The political consequences of seeking to bring the two extremes in Northern Irish politics together were equally pernicious, resulting in the virtual annihilation of the representatives of moderate mainstream opinion, the Ulster Unionists and the nationalist SDLP.

When the first power-sharing assembly was formed, it was the Democratic Unionists and Sinn Fein who divided the spoils among themselves. The spectacle of Martin McGuinness and Ian Paisley — so comfortable in each other’s company that they were nicknamed the Chuckle Brothers — running Northern Ireland was another hard-to-believe moment in the recent history of the province.

Sinn Fein, however, still has pan-Irish political ambitions, though it has struggled to attract supporters in the south in recent years. After nearly a century as a separate nation, the anti-British rationale that sustained the republicans in the north faded long ago south of the border. When McGuinness stood for the Irish presidency in 2011, he gained just 14 per cent of the vote.

Last night at Windsor Castle, he sat among the great and the good of the British Establishment, including Sir John Major and General Sir Nicholas Houghton, Chief of the Defence Staff. Had he won, of course, it would not have been President Higgins sitting beside the Queen, but President McGuinness.

The Telegraph Group Limited, London 2014