With appropriate pomp and pride, Britain celebrated the bicentenary of the Battle of Trafalgar last week with a look back at the achievements of a Navy that can trace its origins as far back as Alfred the Great. Nostalgia for the days when Britannia ruled the waves is always a danger for the Senior Service. But the latest attempt by the Admiralty to predict the future needs of the Royal Navy is admirably clear-headed in recognising that any naval force will be needed largely to support soldiers in land wars.

The admirals have even come up with a jarring new acronym — VMF — to describe the new mission, the Versatile Maritime Force. Where they failed, however, is in their vision of a force that will remain a blue water navy, capable of operating anywhere in the world. The blue water should be between this lingering, anachronistic concept and the role of ships in the complex irregular warfare with terrorist enemies that now poses the main threat to Britain’s security.

Today’s Navy is small, comprising 32 warships, 15 submarines (including four Trident ballistic missile vessels), 97 frontline aircraft, one Royal Maritime Commando brigade and a total of 36,000 people. Under current plans it is due to be even smaller, with the number of frigates and destroyers reducing to 25. Vast sums will be needed if even this small number is to be kept up to date. The main cost will be for two new carriers, to replace the two ageing ships currently in operation and the one now mothballed.

Carriers are hugely expensive. But both the Americans and the British recognise that they will remain essential in providing mobile bases for the aircraft to support future land operations. Carriers, increasingly, will substitute for costly and politically controversial bases overseas. And the modern versions, formidably armed with missiles and protection systems, are less vulnerable than they used to be to aerial or torpedo attack.

The problem, however, is not only cost but also predicting the future. To justify such enormous investment, the Navy claims to be looking at strategic needs for up to 50 years. It is a tall claim. Who can foresee the military balance in even ten years? What will China’s force projection be? What will the threats be to the Gulf, the Middle East and British strategic interests?

Matching maximum flexibility with continued global reach is over-ambitious. Only a decade ago, European navies had scrapped almost all minesweepers; now these are seen as vital. Replacing Trident is a separate issue and one for the Government to decide. Any decision on this will affect naval priorities, however. The new versatile force must also continue to consider the role of submarines and whether so many will still be needed.

The Navy understands that the days of purely naval engagements are probably over: those in the Falklands were the last. But it seems still to be lobbying for the assets to deploy in force anywhere in the world. In any inter-service competition for resources, it must, and will, take third place. As a support force, however, it remains vital.