During his trip to Edinburgh last week, David Cameron rather unexpectedly announced that he supported an increase in the powers of the Scottish Parliament. The current devolutionary settlement, he said, did not have to be the "end of the road" and, provided Scots vote to reject independence at the referendum in 2014, he would be willing to examine ways in which it could be "improved further". Over the weekend, both Michael Moore and Alistair Darling expressed similar sentiments, although, like the prime minister, they refused to say how they thought Holyrood's legislative remit should be enhanced.
　
As Tim Montgomerie explained in the Guardian on Monday, there is a clear political rationale to this new "progressive unionism". The reality is that most Scots support greater fiscal autonomy and, so far, attempts to draw a line in the sand at the status quo - or, worse still, the Scotland Bill - have only played into the hands of the SNP. It makes sense, then, for unionists to seize the initiative by embracing federalism - or some variant of it - and handing Scots responsibility over the bulk of their financial and economic affairs. This would undermine the drive toward separation by sating the Scottish appetite for more self-government.
　
But would it? A federal UK would mean Scotland was only just shy of out-right economic independence. It would see Holyrood take charge of, among other things, Scottish income and corporation taxes, national insurance and - in all likelihood - North Sea oil revenues, while foreign affairs, VAT and monetary policy remained reserved to London. Further devolution for Scotland would have to be met with some form of devolution for England. This would almost certainly involve prohibiting Scottish MPs from voting on English-only matters. Under these conditions, the Union would amount to little more than a kind of glorified defence alliance, with Westminster's UK-wide role being restricted to that of conducting Britain's external relations.
　
The difficulty, though, from a unionist perspective, is that the case for Scotland to determine its own foreign and defence policies is at least as strong as that for it to determine its own economic policies.
　
For instance, an independent Scotland could cut its defence expenditure from the £3.1bn it currently contributes to the British defence budget to around £1.8bn in line with the Nordic average. This would represent a significant saving at a time when public finances were under considerable pressure. It could also force the removal of the hugely dangerous yet strategically redundant Trident nuclear missile system from its waters, thereby substantially improving its security situation. Finally, it could fashion a new role for itself in international politics which reflected its status as a small, northern European social democracy, rather than remain anchored to the UK as it struggles against the decline of its global influence.

Currently, these arguments do not chime with majority opinion in Scotland. But then, a decade ago, the idea that the Scottish Parliament should raise most or all of the money it spends didn't chime with majority opinion either. What changed was Scots' sense that they were capable of governing themselves: the more they did it, the more they wanted to do it. This bears out the "slippery slope" theory advanced by people like Tam Daylell and Michael Forsyth, the most staunch defenders of the UK's unitary political structure. They warned that, as Ian Macwhirter puts it, "independence is a process, not an event" which will occur incrementally over a number of years and through a series of different devolutionary stages, whether people vote for it directly or not. In light of recent events, it is becoming increasingly difficult to say they were wrong.

So, although Cameron, Darling and Moore may view federalism - or devo-max - as the best way to preserve the Union, there is a strong chance it actually represents another step along the road to Scottish independence. Devolution has a logic and a momentum of its own. So far it only seems to be weakening the ties which hold the UK together.