Leading article: A fascinating battle begins

We can see why David Cameron chose to confront Alex Salmond, First Minister of Scotland, last week. The Prime Minister recognised that he would have to grasp the thistle at some point, and it would be better to do it early, and at a time of his choosing, than to be forced into it later. However or whenever he did it, Mr Salmond would find a way of portraying him as a colonial master telling the bonded slaves about changes in their work rotas. That said, Mr Cameron did not handle the Government's decision to facilitate a Scottish referendum brilliantly. Lord Forsyth, who as Michael Forsyth was the Conservative minister associated with using Scotland as a laboratory for the poll tax, should have been dissuaded from making any media appearances.

As so often, Mr Salmond's reaction to the decision was adroit. He left the public response to Nicola Sturgeon, his deputy, as if dealings with a foreign power were beneath his dignity, and then took the headlines with his announcement of the date of the referendum: autumn 2014. With message discipline that would make Alastair Campbell blush, his MPs accused the Conservatives – rather than the coalition – of "interfering" in Scotland's affairs. If the Liberal Democrats were mentioned, they were presented, along with Labour, as part of a three-party conspiracy against plucky little Scotland.

By naming the date, Mr Salmond has lost a little room for manoeuvre. But his Scottish National Party had promised a referendum in the second half of this 2011-15 Scottish parliament. First blood, then, to Mr Salmond.

By the end of the week, something big has changed. Mr Cameron has conceded that the Scottish people have the right to decide their future and that the rest of the United Kingdom will respect that decision, and we know the date of the referendum, even if we do not know exactly what the question will be.

The First Minister has thought about the question of the question long enough, we suspect, to realise the advantages of putting maximum devolution in play. As Professor John Curtice explains today, "devo-max", devolving tax and benefits but not defence and foreign policy, could be a further stepping-stone to independence if the ultimate goal cannot be achieved in one go.

Mr Cameron hopes that an either/or referendum will force the SNP to argue for independence on its merits, rather than relying, as he sees it, on the politics of grievance. The Prime Minister thinks that this is an argument that he can win, and seems to be labouring under the illusion that a "No" vote will settle it for good. His chief strategist, George Osborne, seems to believe that the Scottish people can be frightened by the warning that the Bank of England would not stand by a Scottish currency.

On the other hand, it is more likely that last week's change will set the seal on a trend flowing in Mr Salmond's favour since devolution was agreed in the referendum of 1997, which turned out to be not a "settlement" at all but the start of a process. Since the SNP formed a minority administration five years ago, it has cohered into an effective fighting force with a better calibre of leader than its Scottish rivals. Another unintended consequence of devolution has been to expose the hollowed-out weakness of Scottish Labour as a pork-barrel machine with no engine. The Scottish Liberal Democrats and the Scottish Conservatives have also declined, unable to renew their long and venerable traditions.

Into that space – briefly filled with greens, Scottish Socialists and other varieties of independent – the SNP has expanded. Since 2007, the party has handled the responsibility of office with sufficient skill to make its leadership of an independent country seem much less of a change or a risk than it used to be when a vote for the SNP was seen as a protest vote.

Thus the burden of proof that Mr Cameron hopes will fall on Mr Salmond's shoulders may instead be borne by the defenders of the Union. In that campaign, fear – of the euro, of debt, or of defencelessness – may not be enough. Nor will windy rhetoric about being stronger together than apart be sufficient. In the two-and-a-half-year referendum campaign that starts now, it is up to those who value the Union to present the positive case to the Scottish people that they would be better off in the UK. Let battle be joined.