Gerry Adams and Ian Paisley first came into each other's orbit in 1966, on the 50th anniversary of the Easter Rising. Sinn Féin had defiantly displayed an Irish tricolour in its office window in Belfast and Paisley threatened to lead a march to remove it unless the authorities did so first. Such a display was illegal at the time. The police moved in, removed the flag, and republicans promptly reinstated it. The RUC smashed down the door to remove it again. Fierce rioting broke out and 350 police officers roared up with armoured cars and water cannon to restore order. It convinced the young Gerry Adams that "the north of Ireland was a state based on the violent suppression of political opposition".
The lines were drawn for decades of bloody conflict, a conflict that now stands on the brink of being finally resolved with the astonishing prospect of Paisley's DUP and Adams's Sinn Féin sharing power in a devolved government. If that happens, it will mark the triumph of the extremes of loyalism and republicanism, which have marched over the debris of their moderate political rivals in both camps. It would also crown Tony Blair's decade of endeavour to solve the Irish Question. If success is achieved, it will rank high in the Blair legacy - although it is unlikely to overshadow the chaotic legacy of Iraq.
But we're not there yet. The republican movement has delivered its part of the bargain. The IRA has decommissioned its arsenal and Sinn Féin declared its support for the reformed Police Service of Northern Ireland, turning on its head almost 90 years of violent hostility to policing in the North. Remarkably it did so with only a marginal 5% dissent.
Adams has declared "the war is over", and Paisley knows that it is - at least in the military sense. He's far too long in the tooth to believe that republicans have set aside their historic goal of a united Ireland and knows that they will continue to pursue that holy grail for which so many of their comrades have died.
The irony is that Paisley may be about to become prime minister himself. If so - given declarations down the decades that he would never, ever surrender - how will he spin such an extraordinary turnaround to supporters who have long worshipped him as the rock on which Protestant Ulster and the union have stood?
First, Paisley could argue that through standing firm and refusing to compromise, he has finally defeated the IRA - and that the enemy, at least the armed version, is no more. Second, that the union is secure - ironically made so by the Good Friday agreement their leader rejected. Sinn Féin has accepted that Northern Ireland will remain part of the UK as long as that remains the wish of the majority of its people. And that means it is likely to do so for a long time.
The intriguing question is: will Paisley declare such triumph from the steps of Stormont and the world's television studios to rub it into the noses of his partners in government? Perhaps he will hold back: Paisley, seen by many as the monster of old, now the peacemaker and statesman. We'd still have to blink to believe it. It would be wrong to think that the "Big Man" has changed. He has not. His instincts and his Free Presbyterian faith remain strong. If he finally does the deal and becomes the province's prime minister, he knows that the enemy is now within and he has no illusions about its determination to pursue the goal of a united Ireland.
Paisley helped politicise the young Adams on the Rising's 50th anniversary. Prime Minister Paisley will do all he can to make sure Adams does not achieve his goal by its centenary in 2016. A marriage in government between Sinn Féin and the DUP there may be, but the going is likely to be rough.
Peter Taylor, a BBC reporter who has covered Northern Ireland for over 30 years.