The U-turn shocked everyone.

Brakes shrieking and tyres burning, the small blue and white car spun on its axis and shuddered to a halt facing in the direction from which it had come. It was the biggest about-turn since Steve McQueen spun his V-8 Ford Mustang in the movie Bullitt. Onlookers were shocked and dismayed by the sheer capriciousness of what they had seen. The anger would come later.

Meanwhile, in the real world, that was roughly what happened when the SNP announced that they would end decades of opposition to the Nato alliance by allowing the question of membership for an independent Scotland to be debated at the party's autumn conference in Perth.

The motion will be introduced by defence spokesman Angus Robertson and First Minister Alex Salmond has already announced that he will vote in favour of its adoption. It promises to be the mother of all debates. On the face of it there is nothing particularly wrong with the notion that Scotland should join the transatlantic alliance. In today's uncertain world there is safety in numbers and Nato's concept of collective security is a sensible option for small countries such as Belgium, Latvia and Norway which are already members.

Even the Westminster Government has acknowledged that fact. When Defence Minister Philip Hammond announced his latest swingeing cuts to the British army he acknowledged that future military operations could only be mounted in coalitions or with a larger and more powerful partner. A glance at Nato's order of battle in Afghanistan reveals that all of its 28 independent member countries are pulling their weight in current operations and that even the Davids are out there among the Goliaths.

Among the smaller Nato countries one finds Estonia (153 troops), Slovakia (331 troops) and Slovenia (89 troops). Denmark's contribution is 692 troops and, as this is the country most often used as a point of comparison with Scotland, it is probably true to say that this would also represent Scotland's input as a Nato member – roughly the size of an infantry battalion.

So, nothing wrong there; it would be an equable repayment for the right of membership and acceptance of the rules. By becoming a member Scotland would sign up to Article 5 of the North Atlantic Treaty which determined that an attack on one member represented an attack on the alliance's collective membership.

It has only been invoked once – on September 12, 2001, after the al-Qaeda attacks on the US – but it remains a central principle. All for one and one for all.

It works too and helps to explain the presence of so many Nato troops in Afghanistan but it's not always popular with those who pay the bills for the deployments – the taxpayers of the countries involved. Earlier this year, during the French presidential elections, polls showed that 84% of the public wanted a French withdrawal this year and the new leader, President Francois Hollande, has already vowed that he will "ensure our soldiers come back before the end of 2012". My guess is that a majority of Scots would feel pretty much the same way.

Leaving aside the weighty matter of whether it is a good or a bad thing for Scotland to be a cog in the world's largest military alliance, the question of membership will open a whole can of worms about the SNP's attitude towards the possession and possible use of nuclear weapons.

Mr Salmond has attempted to evade the issue by claiming that the decision will not compromise his party's anti-nuclear credentials and that "the non-nuclear position of the SNP is paramount" but facts are facts. Like it or not, whether it be a matter of perception or the facts of life, the reality is that Scotland will find itself embroiled in a wider debate about the future of the UK's nuclear deterrent by joining Nato.

Three months ago in Chicago the alliance reviewed its current defence and deterrence posture and arrived at the view that "nuclear weapons are a core component of Nato's overall capabilities". At present only three countries provide that capacity – France, the UK and the US – but five other members are involved in the policy of "nuclear sharing" under which they could be given wartime use of some of the estimated 180 US nuclear free-fall bombs stored at US bases in their territory. These countries are Belgium, Germany, Italy, Netherlands and Turkey. And let us not forget that, throughout the Cold War, West Germany was a veritable nuclear arsenal.

Under present SNP policy, after independence the Westminster Government would be required to remove its nuclear weapons from the Clyde estuary – the four Vanguard class submarines from the base at Faslane and the nuclear warheads from the Royal Naval Armament Depot (RNAD) at neighbouring Coulport.

This sounds easy but it will not easily be achieved and most senior naval personnel argue that it will take many years to decommission RNAD Coulport and move its contents south. During that process it would not be unthinkable for Nato to attempt to impose the concept of "nuclear sharing" or even for Westminster to argue for pooled defence in much the same way that the Royal Family might become a collective entity if Scotland votes Yes in 2014.

Quite rightly, as a newly independent leader Mr Salmond would stoutly resist any such move and could also argue that "nuclear sharing" violates Articles I and II of the Nuclear Non-Proliferation Treaty, both of which prohibit the transfer of direct or indirect control over nuclear weapons. But it would be a long-drawn out legal process, as was revealed by the 2009 Edinburgh conference on Scotland's nuclear obligations under international law.

Perhaps in a pre-emptive strike to counter the issue, Mr Salmond also announced last week that he would be prepared to trade-in the weapons on the Clyde for "something more useful". He did not reveal what that might be but many Scots might say that it could be any one of the following – the upgrade of the A9, more hospital beds and increased investment in the industrial infrastructure.

If only. The odds are that he wants to exchange Trident for cruise missiles and other conventional weapons which would be fair enough if they did not also kill and maim. And here's a historical paradox: nuclear weapons have not been fired in anger since August 1945, whereas conventional weapons are constantly in use.

Joining Nato would certainly be a privilege if that was the accepted will of all Scots but we would soon find that privilege brings responsibilities.