Today MPs are debating the 10th anniversary of the decision to go to war in Iraq. For most of us, this anniversary of the US-led invasion is largely history, but for the people of Iraq, it's a state of continuing war. Bombings, murders and other acts of terrorism continue on a daily basis. Tensions are growing between the Shia Muslim majority and the minority Sunnis, raising fears of a return to the worst levels of sectarian violence.

Just this week, there were harrowing reports of at least 70 people killed in a single day, while last month was the bloodiest since June 2008, with more than a thousand Iraqi civilians and security officials killed. And even if Iraq finds a way out of its current difficulties, there is the legacy of the past decade of warfare and terrorism to deal with.

In today's debate in the House of Commons, MPs will reflect not only on the war itself, but on the parliamentary process that led to it. For the 179 British service personnel who have been killed, the many more who have suffered physical and mental trauma, and the hundreds of thousands of Iraqi men, women and children who have lost their lives, this is the least we can do.

About two-thirds of British people now say that the Iraq war was a mistake. Ultimately, parliament was responsible for the decision to go to war. So if this war was a mistake, what should parliament do now?

If it were a public body – a school, hospital or a local authority – we would expect an admission that things had gone wrong, and a pledge to learn the lessons so that it could not happen again. Among the media, public and MPs, a commonly held belief is that people were simply misled into supporting the war. Plenty of evidence shows that the case for war set out by the Blair administration in 2003 was deeply flawed. Intelligence was misused, concerns raised by experts suppressed, and the legal and political position misrepresented. People say that if they had known then what they know now, they would never have supported the war.

Documents from the time do show that there was a deliberate intention to mislead. When Tony Blair decided to join the US in invading Iraq and removing Saddam Hussein, he knew that the public and MPs had doubts. So he used every opportunity to twist the evidence to isolate his critics and encourage his supporters. Britain was, indeed, spun into war.

But this isn't the whole picture. The fact that some MPs were not taken in by the spin exposes the reality that others could easily have known then what they know now. Nearly all of the evidence available now was in the public domain at the time. By asking the right questions, trawling through the documents, some MPs did see through the lies and deception, and stood up in parliament to state the war was based on a false prospectus.

Chief among these was the former member for Livingstone, Robin Cook. In his resignation speech, delivered with the eloquence and authority of a former foreign secretary and cabinet member, he articulated his damning critique of the government's position: why war was unnecessary and unjustified, and exposing the misinformation and deceit.

Yet his warnings were ignored. There are a number of reasons given as to why, but most come down to the idea that the government knew much more about the risk posed by Saddam Hussein than they could say. But in a way, it is more simple than that: most members put loyalty to their leaders and their party above their own judgment, swallowing their private doubts, accepting what they were being told, and voting accordingly.

Loyalty can be an admirable quality. There are times when it is right to bite your tongue, go along with the majority, set aside your own opinions and accept the experience of others. But there are also limits. Committing our country to war, asking our servicemen and women to fight, accepting that people will die in our name, must be beyond the sway of party loyalty. Like capital punishment, like abortion, war is a matter of conscience.

That is why, when MPs are faced with a decision about whether to go to war in future, we cannot have a situation where the government of the day tells the story and we are expected to take it at face value.

If parliament is serious about learning the lessons of Iraq, then it must establish a constitutional convention that votes on going to war are not subject to party whipping. In any future vote, we and our successors must be satisfied that through the evidence that we have heard and seen, the case for war has been made. Three lines on a whip sheet are not enough.