Events in Basra are an alarmingly graphic illustration of the dangers faced by British troops. The television pictures of unrest, angry mobs and British soldiers fleeing for their lives with their uniforms in flames will stick in the minds of the British public for a long time to come.

The debate is now shifting from the calm of Westminster to the violence of Basra; from the carefully crafted assurances of progress from ministers in London to the turmoil in the streets of Iraq. We should not be surprised. For some months, British forces have had to contend with increasingly organised Shia militias, who are strengthening their influence in southern Iraq. Tragically, six British soldiers and two security guards have given their lives in the past eight weeks.

We should not question Army commanders for strong action to secure the safety of the two men who were being detained. If they had reason to believe that the men's lives were in danger, they had an obligation to act. But it is time to ask hard questions of ministers - the questions they seem unwilling, or unable, to answer.

What action will we be taking to respond to increasing hostility and violence in Iraq? What is the assessment of levels of violence between now and the referendum on the constitution on 13 October? Are there plans to send more British troops to Iraq? What effect would an increased deployment to Iraq have for our obligations in Afghanistan? Can we find the 5,000 troops due to be sent there in March next year? When and in what circumstances will British troops in Iraq be brought home? What is the exit strategy?

This was an illegal war, on a flawed prospectus, without the authority of the United Nations. And every leaked document confirms that judgement. But once we had invaded Iraq, the United Kingdom had an enforced moral obligation to the people of Iraq, to help to rebuild what we had destroyed; and to work towards stability and democracy.

The British and American governments have failed in the fulfilment of this obligation. The misjudgement of war has been matched by the mishandling of occupation. Rudimentary principles of post-conflict stabilisation, state-building and reconstruction have been ignored.

Now the insurgency threatens the unity of Iraq. The risk of dismemberment of Iraq increases, civil war is in prospect and instability threatens the whole region.

British forces have served with courage and distinction in Iraq. But we have an overwhelming obligation not to expose British forces to unnecessary risk; a degree of risk so vividly exposed by events in Basra.

And we have obligations, too, to the people of Afghanistan. Despite last weekend's elections, much of the country is dangerous and ungovernable. Promises to deliver stability and human rights, made nearly four years ago, remain unfulfilled.

President Hamid Karzai continues to call for more troops and more aid for reconstruction. Last year the Government announced reductions in the size of the British armed forces. They could not have been more badly timed.

What is required now more than ever is a clear and coherent exit strategy for Iraq. In testimony before a parliamentary committee on 8 February, the Prime Minister said his Government would publish a paper setting out "what the next steps and over what period the Iraqiisation of security will take place (sic)." No such document has been published. It is long overdue. The British public are entitled to know by what benchmarks the Government will judge that we have fulfilled our obligations to the people of Iraq.

I am not arguing for an immediate withdrawal of troops. But as the Foreign Secretary himself has recognised, coalition troops are at the same time part of the solution, and part of the problem.

Between now and the constitutional referendum, all efforts must be made to incorporate Sunni Arabs into the political process. The priority must be to repair sectarian divides and to arrest the slide towards civil war.

We must improve the effectiveness of measures to train and equip Iraqi forces. We must redouble efforts to deliver public services, such as water, electricity, sanitation, and health care, which in some parts of Iraq are worse today than they were under Saddam Hussein. And crucially, steps must be taken to strengthen the sovereignty of the Iraqi government, so that Iraqis can be seen to be in control of their own affairs.

We remain the prisoners of events. The Government must bring clarity to its policy on Iraq. Nothing less will do.

The author is deputy leader of the Liberal Democrats