A man once offered to sell me an AK-47 assault rifle for £6.50. It was no bargain. We were in Somalia; in some African countries, you can pick up a Kalashnikov for less than £4. War is cheap. Peace is a lot dearer. And yet there can be no development without peace. So David Cameron, on the face of it, was only talking sense when he said he may use aid money to pay the British Army to do peace-keeping in poor countries.

Much cheering from Tory backbenchers, who like a bit of red meat tossed their way. And it doesn't come much redder than taking cash from the world's poor to top up defence spending.

But if there is no development without peace, there is also no peace without development. War does not just kill people. It destroys roads, bridges, farming equipment, telecoms, water and sanitation systems. It burns hospitals and schools. It retards trade and economic life, or completely halts it. War tears asunder the fabric of society and creates havens for international terrorists

So the first prerequisite of sound development is preventing war in the first place. When we were writing the report of the Commission for Africa, we analysed a number of studies which showed that conflict prevention is far more cost effective than military intervention. A mere $1.5bn could have prevented the outbreak of fighting in Somalia compared with the $7.3bn it cost the West to respond.

What the Americans call the "CNN factor" means politicians act only when the global media arrives somewhere where things have got out of hand. Too late. More unglamorous earlier action does not attract headlines, but it is far more effective and hugely cheaper.

The irony is that Britain's Department for International Development has been a world leader in such preventative measures through the very kind of aid Mr Cameron now proposes to cut to fund the Army.

Much has been learnt about what works – and what doesn't – in aid over the past three decades. As a result, there has been a massive shift to focus on what actually reduces poverty – so much so that the 2002 International Development Act makes it illegal to use aid only to boost British businesses or for other political purposes.

Over the past 15 years, British aid has been targeted on measures to remove the preconditions for conflict. It tackles the inequalities and exclusion which are the structural causes of conflict. It supports reform in the police, the independence of the judiciary and the ability of communities to hold governments to account.

It has attempted to control the flow of the small arms. It is promoting greater transparency over the high-value commodities which people fight over – such as diamonds, oil and coltan.

What critics of aid fail to point out – or don't know – is that studies of what works have been particularly critical of the joint Conflict Pool through which the Ministry of Defence, the Foreign Office and DfID co-operate. This, ironically, is what Mr Cameron, now wants to boost. Yet a recent evaluation by the Independent Commission for Aid Impact flashed a highly critical amber-red warning over the Conflict Pool, raising serious concerns about its limited effectiveness in creating peace – and its managing and monitoring of public money.

The truth is that peace-building is a better and cheaper strategy than peacekeeping. Attempts to twist aid spending to boost the military will fall foul of the international OECD/DAC rules on what official aid may be spent on: the delivery of humanitarian aid by armed forces is allowed, as is mine removal and election monitoring, but not military equipment or peacekeeping enforcement or anti-terrorism activities. Demobilising troops in Afghanistan would be OK but not troop training in Mali or providing assistance to rebel fighters in Syria.

There are other problems. Blurring the distinction between soldiers and aid workers can lead to dangers for both, as attacks on aid workers by the Taliban have shown. And critics of "Britain's bloated aid budget" conveniently ignore the fact that the nation already spends almost five times more on defence (£37.2bn) than we do on aid (£7.9bn). Nor do they point out that what is really distorting the defence budget is the cost of renewing Trident and two new large aircraft carriers – not the equipment and expertise needed for peacekeeping operations.

Mr Cameron has thrown a bone to his baying backbenchers but, in reality, even switching a billion from aid would not have much impact on future defence spending. As a prime minister unlikely to serve a second term, he would be better going down in history as a leader who kept his promise not to make the world's poor pay for the economic crisis caused by greedy Western bankers.

The steady alleviation of poverty among the world's most grindingly poor people is a noble legacy. He would do better to preserve that than to find out that the bone he has thrown turns out to be a boomerang.