THE Prime Minister chose an opportune moment to make one of his relatively rare forays into Scotland yesterday.

With the First Minister away on his travels and some alarming nuclear-fuelled sabre-rattling emanating from North Korea, there is not going to be a better time to talk about the nuclear deterrent.

David Cameron knows this is a tricky subject, with polls showing four in five Scots keen to see the back of Trident. That is why the SNP would like, with some justification, to turn debates about defence policy into an argument about Trident.

Mr Cameron chose his approach carefully, appealing to popular patriotism by using the return of HMS Victorious from the 100th Vanguard patrol to praise the dedication of the submariners and the sacrifice of their families. He also made a rational case for the Trident replacement, without getting into arguments that need to be had about whether the so-called independent nuclear deterrent is either independent or a deterrent.

Labour and the Liberal Democrats look to be heading for an unconvincing fudge on a cut-price version but the Prime Minister is not alone in doubting that there are cheaper ways of providing a credible alternative.

Though his statement that Pyongyang soon could target Britain with its nuclear missiles smacked of Tony Blair's overblown 45-minute claim regarding Saddam Hussein's weapons of mass destruction, he has a case when maintaining that the nuclear threat did not disappear with the Cold War. Nuclear weapons cannot be disinvented. Those who favour unilateral disarmament must explain how the west would defend itself if countries such as the UK, the US and France dismantled their weapons, leaving regimes such as Iran and North Korea to develop and share them with other unstable regimes. Better, surely, to negotiate them away than give them away. The fact that three-quarters of Scots would favour an independent Scotland's continued membership of nuclear-armed Nato suggests some confusion on this issue. Despite the SNP's U-turn on membership, it is far from clear that Nato would be happy to welcome an independent Scotland without Trident. Expecting others to provide Scotland's (or the UK's) nuclear deterrent is not a morally defensible position.

Mr Cameron also visited defence manufacturer Thales in Govan to emphasise the importance to the Scottish economy of defence jobs. Could a newly independent country afford new frigates and submarines? A country's defence depends not only on having advanced warships but also on the ability to build them. Despite SNP claims of "scaremongering", it seems unlikely that a Westminster government would be prepared to commission complex modern warships from an independent Scotland and, if it did waive EU tendering exemptions, the Clyde would have to compete with shipyards in Poland and France for orders. Could it win them?