It's also a useful counterbalance to Enoch Powell's dictum that all political careers end in failure. For I suspect that Powell didn't just mean failure in the sense of making an utter cock-up; but also failure in the sense of history and circumstance combining to prevent you from delivering your genuinely preferred option. You can only judge a political career by comparing the starting point and finishing point.

Reality

Had anyone suggested that the Ian Paisley who lobbed snowballs at the fairly anodyne Sean Lemass in 1963 would be co-governing Northern Ireland with an unapologetic IRA supporter in 2007, they would have been laughed at or sued. But that is the reality – a reality that still seemed unlikely as recently as the summer of 2006. The two fixed points by which historians will judge him are when we first really noticed him as the snowball-chucking minister of 1963, and the chuckling First Minister he became 40 years later.
The question we have to ask is this: Why did he cut the deal in May 2007? There are, unsurprisingly, a number of possibilities. There is now no doubting the fact (although it was strenuously and misleadingly denied at the time) that he was very seriously ill a few years ago. Indeed, he was closer to death than many key figures in his party actually realised.

Recovery

His recovery – which those close to him hailed as miraculous – may have mellowed him. It would certainly have made him aware of the fact that time wasn't on his side if he wanted to ensure that his legacy would be viewed in a kindly and positive light. And legacy and the judgment of history really do matter to political leaders – particularly those who have spent most of their career on the sidelines.
Or it may be that he really does believe – or, at the very least, was led to believe – that the St Andrews Agreement was a much better deal than the 1998 original. It's a line that he and his successor have been trumpeting for a year now, even though most of the available evidence would indicate that it's a thoroughly disingenuous line. While I'm prepared to admit that the DUP was able to nail down a number of issues (which would have been nailed down earlier had they bothered to stay in the process in 1997), the fact remains that it missed most of the main opportunities for reforming and beefing-up the Belfast Agreement.
Again, perhaps Paisley concluded that there would be something very much worse around the corner if he didn't strike a deal as quickly as he could. He admitted as much in an interview with Stephen Nolan last year, when he spoke about Plan B, late night calls from the Secretary of State and his concern that it would be "curtains" for the Union if he didn't agree to cooperate with lifelong opponents. In other words, he put the needs of Northern Ireland above his own personal opinion and sold the unacceptable to his party. How very noble, you might be tempted to think, were it not for the fact that conversion to the deal with Sinn Fein happened after, rather than before, the 2007 Assembly election.

Alternative

During and after the 1998 referendum the DUP had been promising that there was an alternative to the Belfast Agreement: and that alternative lay in shredding the original and negotiating a new one. By December 1999, when d'Hondt was first triggered, the DUP became part of a government which included Sinn Fein and the first hint of the "fairer Agreement" approach began to creep into their speeches and strategy. By early 2004 – a few weeks after eclipsing the UUP in the 2003 Assembly election – the DUP was letting both the British and Irish governments know that they would accept the reality of Sinn Fein in government with them.
Put bluntly, the DUP had accepted, and from a very early stage, that it was not going to be able to replace the Belfast Agreement at all. The best it could ever hope for was that some tinkering around the edges (accompanied by the fact that the UUP was still having huge internal problems) would convince enough unionists to trust them – and Ian Paisley in particular – to deliver something better. That the something better would include 95 per cent of the Belfast Agreement and a doubling of Sinn Fein's representation on the Executive was, it seems, an acceptable price to pay for hiding the reality that the DUP had been rolled-over and comprehensively mugged at St Andrews.
Or maybe, just maybe, the DUP's decision to cut the deal with Sinn Fein was based on nothing higher than hypocrisy and naked opportunism. Having spent 35 years nipping at the heels of the UUP, accusing them of Lundyism, treachery, spinelessness and weakness, the DUP ended up operating the same structures and co-operating with Sinn Fein on a more extensive scale than the hated UUP. Bearing in mind that there is precious little to show for this change of heart and that all they are doing is shoring up mandatory structures which they had once described as "unalterably anti-Unionist", isn't it fair to conclude that the DUP has simply bottled and opted for authentic mammon?

Take your pick

Take your pick from those five possibilities; or add a few of your own and write to the Letters page. We may never know the real reason, unless Dr Paisley puts pen to paper and explains the conversion. And even then it may simply be a sanitised and self-justifying explanation.
And my own view?: As early as the summer of 1998 – and I wrote about it at the time – the DUP knew that it could never go back to the drawing board and rewrite the Agreement. It pretended (and I use that term in its most precise sense) that it had a strategy for delivering something better and banked on the fact that the UUP would be put through the mill by Blair, Sinn Fein and general circumstance. All it had to do was wait until Trimble had taken the hits and Ian Paisley had arrived in the driving seat; then tie up a few loose ends and snatch the benefits. Some may describe that strategy as utterly, thoroughly, comprehensively hypocritical. I couldn't possibly comment!