Leading article: The starting gun is fired in defence of the union

David Cameron's visit to Scotland yesterday was more than just a routine heave in the tug-of-war with Alex Salmond over Scottish independence. As the first time that the British Prime Minister has set out the case in favour of the union, Mr Cameron's speech in the shadow of Edinburgh Castle was a signal moment in the debate. It was also as tough a test of his political dexterity as any yet.

The Prime Minister certainly had plenty of ground to make up. He was largely silent on the question of Scottish independence until last month, leaving Scotland's tenacious First Minister to dominate for the first 18 months of the Coalition Government. In fairness, the Conservatives are not the only ones to have ducked the issue. Neither Labour nor the Liberal Democrats has taken any more of a lead. Both must do so as the debate intensifies ahead of the ballot proposed for 2014.

The topic is far trickier for Mr Cameron. Although it is true, as the joke goes, that there are more giant pandas in Scotland than there are Tory MPs, Mr Cameron is still the Prime Minister of the United Kingdom. It was absolutely right, therefore, that he make the case in favour of the union, in Scotland, himself. Being a Tory, however, rendered the move fraught with risks.

Given Scotland's long-standing antipathy to the Conservatives, a badly judged lecture in Edinburgh risked not only confirming Scots' worst fears about English interference but also undermining the defence of the union. Add in the fact that the Prime Minister himself will not ultimately play an active role in pro-union campaigning, and yesterday's speech became an immensely delicate balancing act.

Mr Cameron's record in government is far from perfect, with a disquieting tendency towards ill-considered, even erratic, decision-making. But after 18 months in Coalition, he has, at least, had plenty of practice at striking a nuanced tone. Judging by yesterday's speech, he has learned the lesson well.

There were, of course, some spec-ifics amid the rhetoric. There was a hint of stick (an independent Scotland would have to negotiate for things currently a matter of right), and also of carrot (with the referendum out of the way, more powers might be devolved). But from the opening references to humility, through the assurances that he has no wish to dragoon the Scots against their interests, to the concluding peroration on the nobility of the "common endeavour" for which the UK stands, Mr Cameron framed Scotland's place in the union as less a matter of totting up pros and cons than one of shared values, of the ties that bind, of the heart at least as much as the head.

It was a carefully calibrated performance for which he deserves some credit. Indeed, it is not just in the matter of tone that the Prime Minister has struck the proper note. We would argue he is also right on the substance.

This is not to say there is no cause for a referendum, given voters' desire for one. And any outcome must, of course, be accepted without quibble and put swiftly into practice. That being said, we share Mr Cameron's hope that the Scottish people will conclude in favour of the union. Not out of devotion to a 300-year-old status quo. Nor even in answer to practical questions over the common currency, say, or North Sea oil. Rather, out of a fundamental belief in the advantages of union over division.

Whether the discussion is of the UK or Europe, the same argument holds. In an increasingly globalised, competitive and uncertain world, the tangible benefits of standing together far outweigh the inward-looking temptations of a retreat behind historic borders. The Prime Minister yesterday set the stage for the pro-union campaign to begin. It is up to Scotland to weigh its case.