Patrick Mercer: Review the review. Our options in Libya are too limited

The only thing certain about war is uncertainty. That's why getting your forces into the right size and shape for what lies ahead is so tricky – and why nations often get it wrong. Add in an island's need for strong maritime and aerial forces and it's easy to see why, from Dunkirk to the Falklands, we've been wrong-footed. One of the givens about Britain's place in modern warfare is that we always fight in an alliance – yet the South Atlantic and, more recently, Sierra Leone showed how uncertain even that can be. Now, just when the latest defence review has been completed and our forces start to be "rebalanced", a largely unpredictable crisis in the Middle East blows up.

No one can doubt the depth of the financial difficulties that Britain faces, and nowhere is that more obvious than in the budget sheets of the Ministry of Defence. The MoD must find savings like every other government department and, with five overblown headquarters to control forces that are smaller than at any time since the Crimea, and more civil servants within it than soldiers, there is plenty of scope. But the MoD is the only department that deals in the business of taking and saving lives, and whose employees have to swear an oath to the boss. That's why the level of risk accepted in the stampede to save money should be revisited.

The careful plans for Future Force 2020 provide "coherent, efficient and cutting-edge Armed Forces prepared for the challenges of the future", that will require "real terms, year-on-year increases in the defence budget in the second half of the decade". But the Middle East crisis is with us now. Less than six months ago, we could have responded to events in Libya and whatever lies ahead with an ageing but capable combination of HMS Ark Royal and the Harrier fleet. Equally, it's a mercy that things went wrong when they did; any later and HMS Cumberland would have been beaten into plough shares.

Some say that future wars will look much like today's, our forces dealing primarily with counterinsurgency operations, probably in hot climates. So fast jets, aircraft carriers, heavy artillery and the like could be sacrificed, in order to move fast and "fight light". But, as the Middle East demonstrates, that would be wrong. The National Security Council's recommendations to invest in a new carrier programme, submarines, transport aircraft and more are welcome. But we need them now.

Plans to reduce the Army to 80,000 from its current 102,000 are similarly misguided. History is littered with similar, noble ideas, not least the wholesale reduction of the infantry in the late Sixties: it had just started to be implemented when Northern Ireland erupted and foot soldiers were needed in quantity. Some thinking about reserves, and the implications of cyber warfare, is sensible, but now is not the time to reduce our pool of ready manpower, nor to stop recruitment.

There have been some very successful defence reviews in the past: Haldane's vision allowed our troops to keep the Kaiser at bay in 1914. Now, we need something similar. The events that stretch from Egypt to Yemen and beyond are, I believe, as cataclysmic as those of September 2001 and our defences must not be dominated by financial considerations. As the signatories to the IoS's letter agree, the Strategic Defence and Security Review needs to be readdressed.

Patrick Mercer served in Northern Ireland and the Balkans. He is Conservative MP for Newark