This week marks 10 years since the start of the Iraq war, a conflict that left hundreds of thousands of Iraqis and thousands of coalition personnel dead. Over one trillion dollars was spent. Yet, a decade on, Iraq remains a nation wrought with instability and division.

The war was a mistake. Recognising that in no way detracts from our patriotism. British armed forces served bravely. Their commitment and dedication – and that of their families – will never be forgotten. Week after week, at Prime Minister's Questions, Parliament would pay tribute to the fallen – 179 men and women in total. It is in their honour, above all, that lessons must be learnt.

The Liberal Democrats opposed the war. My predecessors as leaders, Charles Kennedy and Sir Menzies Campbell made our argument night after night – with the late Robin Cook and others – combining a forensic knowledge of detail with a clear and principled stand. It remains my view, and the view of my party, that the intervention was illegal under international law.

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Long before the first air-raid sirens sounded over Baghdad, the international community was sharply divided. In Britain, one million people marched through the streets. They refused to sit quietly and have the war waged in their name. Courageous individuals – academics and experts, Whitehall officials and military personnel, and journalists, including from The Independent on Sunday – broke ranks to warn of the dangers of armed conflict.

The pretext given by the Blair government for the invasion – Saddam Hussein's possession of weapons of mass destruction – proved false. The intervention led to years of instability, sectarian violence and religious extremism within Iraq and beyond. It strengthened Iran's ability to destabilise its neighbours and it undermined the credibility of the United Nations.

The war in Iraq damaged confidence in the principle of humanitarian intervention and strengthened the hand of isolationists. Tony Blair in his 1999 Chicago speech proposed new criteria for humanitarian intervention. They were almost the right principles – the problem was that he did not follow them.

The failures of Iraq do not alter our collective responsibility to support freedom and protect human rights around the world. Doing nothing can be as bad as cavalier adventurism. The question remains how and when to intervene – and it is always a tough judgement. There are no easy guides for politicians to follow. But, in my view, there are four tests we must always apply. Is intervention legal? Does it command local and regional support? Are we confident intervening will alleviate suffering? And is the UN behind it? Or, in the absence of UN approval, are there reasons to intervene on clear humanitarian grounds.

They are not much different to Tony Blair's principles: the difference is that the coalition government I serve in has stuck to them. In September 2010 I had the honour to give the UK address to the UN General Assembly. I was clear that we had learnt the lessons of Iraq and resolved that Britain would pursue a hard-headed foreign policy based on liberal values. I will be giving our address again later this year and I am proud of our record in the intervening years.

In Libya, we used air strikes and special forces on the ground to remove a brutal dictator but more immediately to spare the inhabitants of Benghazi from a massacre. We achieved both ends, working in partnership with Arab countries and our allies in Europe and the US. At the UN Security Council we secured a strong mandate for military action and the Attorney-General published his advice on the legal basis for deployment of our forces in Libya.

The intervention has not suddenly produced a harmonious and democratic society in Libya. We could never guarantee it would: we are working hard in support of Libya's new government and people, but we are realistic that their future is largely in their own hands. And we have also been the driving force behind EU efforts to respond positively to the Arab Spring as a whole.

Recently in Mali, with the French at the helm, the international community – including crucially many of Mali's neighbours – has forced back dangerous insurgents threatening an impoverished nation and seeking to turn the region into a haven for terrorism. Those tests were met: legal, local and regional support, alleviating suffering and commanding UN support.

Right now, our attention is, of course, focused on Syria. This, sadly, has not been so straightforward. It tests our principles to the limit. Why do we not intervene more boldly in Syria, where civil war continues to unfold? That is a question we must ask ourselves every day, assessing constantly what more we can do and where the balance of risk lies. Until now, the truth is that the UN is divided and we have judged the risk too high that direct military intervention by us or our allies would lead to another Iraq-style imbroglio. Above all, it has not been sufficiently clear that intervention would improve the humanitarian situation.

But we must do – and we are doing – all we can to help those opposing President Assad and to ease the suffering of the Syrian people. We have been a leading donor providing assistance to Syrian refugees. And, earlier this month, the Foreign Secretary announced that Britain, together with our partners, is sending more non-lethal equipment to Syria's opposition. We have rightly refused to rule out going further, if the balance of risks changes.

I am aware of all the arguments against relaxing the EU arms embargo. First, that solutions should be political but not military. But these are not mutually exclusive. Second, the risk that arms end up in the wrong hands. But that is exactly what is appening now. Third, that this might encourage an increase in the supply of arms from Russia to the Syrian regime. But the regime is not running out of arms at the moment – they are being regularly resupplied. There are no simple options, only hard choices. We must be driven by the need to alleviate suffering and avoid at all costs any action that could increase suffering and prolong the conflict. But what we have been doing so far has not worked, and the proof of that are the thousands of Syrians killed and wounded.

Iraq means we have to be alert to the complexity of the situation in Syria, with its disparate opposition groups and rival agendas. But it does not mean standing by. A number of inquiries have been established to get to the bottom of what went wrong with Iraq. Some have achieved more consensus than others. The Chilcot inquiry is intended to provide the definitive answers. Like many, I am frustrated at the delays, and I look forward to the report's publication. When that comes, I am confident the government will respond constructively to any recommendations.

We must continue to operate by the principles I have outlined. We must ensure that our analysis of intelligence is robust and professional and our use of it unbiased. The legal framework we work in must be unequivocal. We must provide greater transparency to Parliament and the public. And we owe it both to our armed forces and to all those who marched against the Iraq war to work soberly and calmly for a more peaceful and stable world.