The Government announced yesterday a full-scale review of Britain’s defence needs. It is a huge task. There needs to be a public debate on the shape of future defence policy.

There is a serious intellectual case for scaling back commitments in line with changed threats and budgetary constraints. But that course should be rejected. In particular, it would be a mistake to abandon Britain’s independent nuclear deterrent in the hope of large savings. The economic dividends would be slight, and the damage to Britain’s defence capability substantial.

The review will be published in two stages. An initial Green Paper next year will assess the broad issues of what defence policy is for and how it is conducted. A wider strategic defence review, with estimates of costs, will appear after the general election. Running through the exercise will be an awareness that the political and economic landscape has changed radically since the last such review, in 1998. First, the most direct threat to security in this decade has come not from hostile states but from terrorist violence on the British mainland. Millenarian fanatics who blow themselves up, along with large numbers of civilians, are undeterrable. Second, the UK economy is labouring under a massive burden of public debt. And third, there is intense public awareness of the human costs of military engagement in Afghanistan, where 173 British soldiers have been killed since October 2001. The argument for scaling back Britain’s military commitments will be heard as the review progresses.

The economic facts are brutal. On the Treasury’s figures, the budget deficit will widen to £175 billion this year. When economic growth returns, there will need to be a sharp fiscal contraction to offset that borrowing. The defence budget will come under pressure. But unlike spending on health and education, defence spending has been controlled. Core defence spending, excluding operational costs, has risen by about half a per cent a year in real terms in the past decade. To curb the defence budget in any substantial way will require the abandonment of large commitments, such as the purchase of two large aircraft carriers and the replacement of Trident, the independent nuclear deterrent. It might mean in practice (in a distinction drawn by Tony Blair in 2007) taking the role of peacekeeping alone rather than peacekeeping and warfighting.

Britain should not adopt that course. There is a direct connection between national security and the ability to project force overseas. The defeat of the Taleban would set back Islamist extremism in the West. Trident is intended to cope with contingencies to the middle of the century. Malign states will acquire nuclear weapons in the meantime. Abandoning a nuclear deterrent would be irreversible. Retaining it would mean that no crisis could run beyond the control of a British Prime Minister owing to a threat of massive force. The capital costs of Trident amount to about 3 per cent of the defence budget. If British security is enhanced this way — and it is — then the cost is far from onerous.

A defence review cannot, however, be a substitute for a searching examination of how the Treasury is financing the war in Afghanistan. Welsh Guards who are in harm’s way today cannot wait till 2010. They must have adequate equipment now.