The United Nations this week will again debate the Middle East. As the diplomats enter its New York HQ they will walk under an Akkadian copy of the “Eternal Treaty”. Signed in 1259BC, it grandly aimed to “establish peace and brotherhood for all time”. In what is often regarded as the first negotiated peace treaty, the Hittite king Hattusili III and Pharaoh Ramesses II concluded two centuries of conflict after the Battle of Kadesh, in modern-day Syria. But although that country may be the cradle of peace diplomacy there’s unlikely to be any need to hammer another nail into those UN walls to hang up a new Syrian agreement.

Against this backdrop, MPs will consider RAF raids against Isis positions in Syria two years after parliament voted on air strikes on Assad. In conceiving this monthly column, I decided not to be a commentator on Labour’s all-too-irregular ups and all-too-frequent downs. I’ve refused three approaches to write a book about what may be the most traumatic six months in Labour’s history. Each offer was financially rewarding but none was in the party’s best interests. But I do want to talk about what happened in that vote on 29 August 2013.

As shadow defence secretary, I knew that what was on the table was a very limited RAF campaign against the Assad government. It was a million miles from regime change by military means. Modern mythology believes that MPs were against the possible use of force. In truth, 270 MPs voted for the government motion and 220 backed Labour’s variant of a similar policy. Both proposed a conditions-based posture on British action – and they had the backing of 490 MPs in total. Labour voted against the government while not expecting to win. The government voted against Labour while not expecting to lose.

That night I didn’t join in the customary cheers of some opposition MPs that greeted the government’s defeat. How could I? We hadn’t just won a vote to protect family tax credits. Assad had dropped chemical weapons on schoolchildren in their playground. Parliament had contrived to do nothing about it. Instead, I had a furious row with Michael Gove as we loudly traded industrial language in full view of dozens of MPs. Over the next few days, I stretched the elastic of collective responsibility to snapping point. I wrote about how uneasy I felt about the outcome and urged parliament to think again.

Yet as time has passed I know I shouldn’t just have written about it. I should have stood down from the shadow cabinet in the hours before the vote. Of the hundreds of votes over 18 years in parliament, 29 August 2013 was the one occasion I allowed commitment to my party to defeat my sense of right and wrong. I should have been true to myself. I will always regret not being so.

Two years on, this summer, prominent politicians were under more pressure over whether they would lend their spare room to refugees, rather than how they would use the power of the house to which they have been elected. Rightly, no government can bind its successor. Nor should any parliament be imprisoned by its predecessor. Of course, much has changed in politics since 2013, not least the presence of the 56 “anti-war” SNP MPs, who will oppose military action anywhere in the world to placate their new members in pursuit of a referendum rerun. And there are others who say it’s now too late to intervene. Many of them are the same people who claimed last time that it was too early to get involved.

I’ve heard the argument that military action won’t work. I agree: alone it would achieve little. An Iraq-type coalition of the willing won’t happen and in any case would fail. What’s more, the idea of a Lebanon-style power-sharing agreement is as naive as Isis is barbaric. The jihadists are too busy beheading innocents and rubbling antiquities to talk. There is no Hattusili or Rames­ses in their ranks.

I respect conscientious objectors and the Quaker traditions. At times in our history it has taken true courage to stand out from the crowd. But conscientious objection isn’t a legitimate posture for a P5 nation in the face of Isis ferocity. And when put on the spot about a Syrian strategy, too many politicians simply parrot the line: “We shouldn’t have attacked Iraq.” It’s a legitimate argument about the past. It’s not a plan about the future.

It all feeds into a sense that, after Iraq and Afghanistan, Britain has re-entered a period of unresolved purpose. After 1956 and the Suez humiliation, the British political class was shaken out of a foreign policy shaped by its past. In 1962, in the immediate shadow of the Cuban missile crisis, the former US secretary of state Dean Acheson told West Point cadets: “Great Britain has lost an empire and has not yet found a role.” In 1968 George Ball, a former undersecretary of state, wrote of a “special problem” rather than a special relationship. He thought Britain had become hamstrung by its history, and bemoaned “Englishmen reared on the heady heritage of exotic empire”.

The most important military decision in the coming months is not Syria but the Strategic Defence and Security Review. The new plan should commit the UK in law to the Nato defence spending target of 2 per cent of GDP. It’s right to enshrine the aid budget in law, and it’s not wrong to do the same for defence.

Britain still has so much going for it internationally. Yet our multinational nation state almost unravelled last year and may yet unwind from Europe. A lack of clarity about what it means to be British at home is matched by huge uncertainty about what Britain means abroad. Parliament should shortly get the responsibility to vote on air strikes. When it does, MPs will not only decide what they believe should happen in Syria, but also what they think of Britain.