By James Robbins
Diplomatic correspondent, BBC News
This really does feel like a "no turning back" moment - offering the best hope of permanent peace for Ireland in more than 400 years.
The period known as The Troubles began in the 1960s
Exaggeration? I don't think so. Most of the people of Northern Ireland want peace and that certainly was not true during many of the years of the worst sectarian killing during the 1970s and 1980s.
During those years, most of the roughly 1.6m people preferred to endure a sort of low-level civil war (always referred to in breathtaking understatement as The Troubles).
They preferred that to the obvious alternative - making compromises with "the other side" to achieve peace.
So, how did we get here?
When I first went to the British province of Northern Ireland as a trainee reporter with the BBC, back in 1978, violence was either at, or close to, its terrible peak.
Both Catholic Republican and Protestant Loyalist extremists were killing with dreadful regularity.
Violence also hit the UK mainland - here at Hyde Park, London, 1982
The Irish Republican Army (IRA) was the most deadly, demanding complete British withdrawal from Ireland and the unification of the six counties of the North with the 26 counties of the South - the independent Republic of Ireland - against the wishes of the majority in Northern Ireland.
It looked hopeless. Britain's secretary of state at the time talked mainly of containment - of reaching an "acceptable level of terrorism".
I spent the best part of five years living in Northern Ireland - reporting on atrocities, attending countless funerals, watching divisions deepen.
The hunger strike by Republican prisoners at the Maze Prison in 1981 looked like a terrible turning point.
The slow suicide of 10 prisoners was so emotionally potent that it forced many people who had previously managed to avoid making hard choices about their loyalties to admit - even if only to themselves - where they now stood.
Within 10 years, however, at the end of the 1980s, it started to look as if the people of Northern Ireland might be so traumatised and exhausted by the violence that they and their leaders might, just might, be ready for a different path.
The Maze Prison. A hunger strike there seemed a key turning point
In the 1990s, everything started to come together. The whole of Ireland was changing.
Two British prime ministers - John Major and then Tony Blair - invested huge amounts of time and political capital in a peace process.
Prime ministers of the Irish Republic were committed. In the US, President Bill Clinton was heavily engaged.
The British government conceded that its army could not entirely defeat the IRA.
Above all, the Republican leadership sensed they were making little headway by reliance on killing. Later, hardline Loyalists would soften their positions too.
Downing St Declaration
Within those first five years of the 1990s, almost everybody, but certainly not quite everybody, started a dialogue.
The British talked secretly to the IRA leadership. In 1992, Ulster Unionists agreed to talk to the Irish government.
Even the famous Good Friday Agreement of April 1998 could only break down some of the crucial obstacles
By the end of 1993, UK Prime Minister John Major and his Irish counterpart, Albert Reynolds, had made the Downing Street Declaration.
It pointed the way to the Republic giving up the claim to rightful sovereignty over the North (enshrined in its constitution).
In 1994, the pace quickened again. President Clinton granted Gerry Adams, the leader of Sinn Fein (the political wing of the IRA) a visa to visit the United States.
Later the IRA announced "a complete cessation of military activities".
US Senator George Mitchell was a key player in the peace process
The potential solution was being internationalised as Loyalists responded with their own ceasefire.
So, by the middle of the decade, the crucial background was in place - if only the extremists could be persuaded to start a process of giving up their guns and bombs, to accept the first stages of "decommissioning" of arms as the price of entry to negotiations.
It took the chairmanship by US Senator George Mitchell of an International Body on Arms, amid a huge amount of ill feeling on all sides and a tortuous process of disarming the paramilitaries, to create the conditions for a possible breakthrough.
Even the famous Good Friday Agreement of April 1998 could only break down some of the crucial obstacles. Almost another decade of fitful and uncertain progress, with huge reverses, lay ahead.
It was not until 2001 that Canadian General John de Chastelain, another key international player, reported that the IRA had a plan to put arms beyond use.
Throughout this whole precarious process the centre parties in Northern Irish politics were being squeezed.
On one side, the mainly Catholic SDLP was being elbowed out by Gerry Adams and his Sinn Fein.
Ian Paisley (left) and Martin McGuinness are sharing power
On the other side, the overwhelmingly Protestant Ulster Unionist Party was losing ground to the hardliners of Ian Paisley's Democratic Unionist Party.
No settlement was ever going to be credible without the radical extremes on board.
That is the key to the future. The most implacable are now working together, sharing power.
Among many other factors which have contributed to peace, I would pick out four at random.
The "supermarket" effect. At the height of the violence, nothing could persuade some of the biggest names in British food retailing, Sainsbury's and Tesco, to risk opening stores in Northern Ireland. As peace seemed a possibility, they both started building there and driving standards of service up and prices down. Ordinary people realised another aspect of what The Troubles had cost them. They do not want to go back.
- The "runaway Republic" effect. The emergence of peace has also coincided with, and been greatly helped by, the astounding economic growth south of the border in the Irish Republic, the Celtic "tiger economy" of Europe. To Unionists, previously deeply fearful of the Republic, this evidence of huge economic competence melted a conviction that getting closer to the Republic would spell collapse for the North too.
- The "secular Republic" effect. The decline of active Roman Catholic congregations in the Irish Republic, as well as the damage done to the reputation of the Catholic Church in the south of Ireland by the scandals of priests molesting children, have combined to accelerate a sharp reduction in the Church's influence over politics. That, in turn, reduced fear among Northern Protestants of "Catholic domination".
- The 9/11 effect. The huge funding for militant Republicanism by Irish-Americans was already in decline before the attacks on the US on 11 September 2001. That day, however, changed the world, and drove many organisations labelled as "terrorist" to the margins. The IRA and its political allies recognised that political violence was suddenly deeply unfashionable among previously staunch supporters.
Peace now has its very best chance in Ireland.