I HAD been out of office on the Friday, inspecting the Combined Cadet Force at Dollar Academy. It was a pleasant enough occasion, standing there in No 1 rig, waggling a shiny sword about for a march-past. Made a change from gloomy Faslane, anyway.

Out in the Gulf, a far less pleasant situation was developing. On the Monday morning I saw a post-it note on my desk. It said simply, “Call ASAP”, followed by a name of an Army Major I did not recognise and a number at Permanent Joint Headquarters, Northwood.

Within 24 hours I was sitting an Arabic exam at an Army camp and kitted out with a pistol and NBC (nuclear, biological and chemical warfare) suit. A day or two later, I boarded an RAF transport plane on an expenses-paid trip to “somewhere near the Iraqi border” – as war correspondents might put it.

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We crossed the Iraqi border by night, just a few days behind the main invasion force. I had always known that doing an Arabic degree might land me in trouble one day, and here I was in it up to my oxters: the only Arabic-speaking British naval officer in a sea of Scud-bunkers and sand.

Suddenly “Oil and war”, which had been my deadpan stock answer to “Why Arabic?” at Edinburgh parties, did not seem so funny.

Before joining the armed forces, one has to wrestle with the morality of it all. For me, it came down to this basic question: Could I, on the whole, trust the state to use armed force in a responsible and morally defensible way? I thought I could. Tony Blair had promised an “ethical foreign policy”. The recruiting slogan was “A force for good”.

Besides, short of all-out war, the Navy’s roles were mostly benign, concentrating on peacekeeping and constabulary duties: protecting merchant shipping from pirates, enforcing UN arms embargos, providing aid to Caribbean islands hit by hurricanes. All this to be followed – I sincerely hoped – by drinks at the Suva yacht club or taking Maltese nurses to wardroom dances. I felt my conscience could live with that.

About the Iraq war I was less convinced. Even without weapons of mass destruction, there was a case for getting rid of Saddam Hussein. In a region filled with authoritarian regimes, Saddam’s was one of the worst. In Mubarak’s Egypt, where I had lived as a student, people could keep their heads down and be basically safe: there was some cultural, economic and civic life beyond state control, the judiciary was fairly independent, and the regime would generally not harass people unless they were active dissidents.

In Saddam’s Iraq, no-one was safe. No self-abasement, no fanatical display of loyalty, was enough to shield people from disappearance, torture or death.

The strength of that case depended entirely, however, on being able to put something better in place. To remove Saddam and leave a power-vacuum would invite civil war and authoritarian fundamentalism – which is, of course, exactly what happened.

If the invasion was legitimate, the handling of the post-invasion situations severely strained that legitimacy.

These ethical qualms soon mutated into constitutional questions: who decided to go to war, on what criteria, how were they chosen, and to whom were they responsible?

Strictly speaking, war-making is a Crown prerogative power, wielded directly by the Prime Minister. Cabinet approval is probably needed under the doctrine of collective responsibility, but there is no legal requirement for a vote in the House of Commons.

That Tony Blair felt obliged to seek authorisation from the Commons – which approved the war by 419 votes to 149 – was in part due to the influence of Robin Cook, whose resignation from the Cabinet forced the issue. Cook’s gravestone reads, “I may not have succeeded in halting the war, but I did secure the right of Parliament to decide on war.”

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Of course, unwritten rules are binding only when convenient. David Cameron respected the new practice of letting the House decide on air strikes against Syria, but Theresa May later ordered the bombing of Syria without a vote.

A proper constitution could regulate these war-making powers, protecting the right of Parliament to decide. Even if the government has a solid majority, requiring a vote means the decision has to be debated; going to war has to be publicly justified and legitimately endorsed.

More generally, keeping the armed forces under the control of the elected civilian leadership is an essential function of any democratic constitution. This week’s coup in Myanmar shows what tragedy unfolds when that democratic control breaks down.

A future Scottish constitution should therefore not only require parliamentary approval for declarations of war or for military action abroad, but also be explicit about the role, purposes, and higher command structure, of the Scottish armed forces. It should make it clear that the Scottish armed forces belong to, and serve, a constitutional democracy in which human rights and the rule of law – including international law – are paramount.

Cathy McCulloch, director of the Children’s Parliament, is the next guest on the TNT show on Wednesday