Sharp-eyed observers outside Downing Street last Monday might have spotted a succession of military academics and generals ushered through the door of No 10.
From King's College London came Professor Sir Lawrence Freeman and Professor Michael Clarke, while from the Royal Marines came Lt-Gen Sir Rob Fry.
They had been summoned by Tony Blair to help him answer two seemingly simple questions: what are Britain's armed forces for and what do they need to do the job?
The answer to the first, delivered aboard HMS Albion on Friday, was, in essence, to make Britain secure by tackling terrorism at source.
But the second question was left hanging in the air, to be answered by Gordon Brown, Tony Blair's Chancellor and successor.
Last September Mr Brown said: "I guarantee we will continue to spend whatever it takes to meet the new security demands we face and our military commitments abroad."
It seems like a commitment to pay any bill presented by the armed forces to Prime Minister Brown - but, just like Mr Blair's celebrated pledge to provide "whatever package they want", a host of devils infest the detail.
For Mr Brown's expected arrival at No 10 this summer coincides with the announcement of the spending settlement for every Whitehall department for the next three years.
Most analysts think he will need to find an extra £15bn a year just to meet the existing commitments to procure new equipment and weapons systems. Improving conditions and pay of service personnel and increasing infantry strength to levels necessary to relieve the current overstretch could incur a similar bill.
Charles Heyman, editor of World Armies, says Britain's defence spending would have to rise to levels last seen at the end of the Cold War - around 3.5 per cent of gross domestic product - to meet all the current commitments.
Sums like these - which would require either tax increases or deep cuts elsewhere - are politically impossible. As Bill Kincaid, of the Royal United Services Institute, says: "The inexorable rise of defence equipment costs will inevitably make our equipment programme unaffordable in the near future. Something has to give - and soon."
Suddenly the future shape of Britain's defence budget is in question. No wonder, then, that when news of Monday's Downing Street "blue-skies" meeting reached the Ministry of Defence, Des Browne and Air Chief Marshal Sir Jock Stirrup, Defence Secretary and Chief of the Defence Staff respectively, insisted on being invited.
One of those present says that Mr Blair asked "one or two searching questions" before each invited expert delivered his case. Soon it became clear that there was a clash of ideas between radicals and traditionalists.
The traditionalists, represented by Mr Browne and Sir Jock, argued for a better-funded status quo. The radicals, represented by Professors Freeman and Clarke and Lt-Gen Fry, wanted the Government to face up to the looming crunch, scrap costly items like the two new aircraft carriers and use the savings to beef up the Army. "I suddenly realised that this was policy being made on the hoof," said one of those present.
Other big-ticket items whose future is being questioned are the new generation of Type 45 frigate, the Astute submarines, and the Typhoon aircraft.
None, sceptics note drily, would be of much use either on the plains of Helmand or the streets of Basra. What is needed is less 21st-century technology and more "boots on the ground".
Most think it is inevitable that Mr Brown will announce a "strategic defence review" when he takes over. The last one, in 1998, pre-dated 9/11. This was widely seen as having laid too much emphasis on quality over quantity and on firepower over intelligence. But once the new review reports, what will Mr Brown's "new model army" look like?
Experts believe he is likely to need slightly more soldiers, and in more flexible formations. Most specialists will have to have a dual role as infantry, to undertake peacekeeping and security, outright war, and humanitarian missions. In all, the UK needs a working ground force of around 125,000. Today the fully trained strength of the Army is around 95,000. The 125,000 could be made up of about 110,000 regulars and 15,000 permanently deployable reservists.
The traditionalists will be hit hard. It must be possible to move men and women between units and career paths with the ease of a big army like that of the United States. "There are too many private armies within our forces," a planner at the MoD said privately recently. One such "private army" is the Royal Marines, which has more than 100 lieutenant colonels, yet fields a force the size of a medium infantry brigade, plus a clutch of small specialist units like the Special Boat Service.
The Navy guards the Marines jealously but it would be more cost-effective to make it part of the Army. The same might go for the RAF Regiment.
The Navy might also have to wave goodbye to the project to build two aircraft carriers of over 50,000 tons. It is now evident it is stretching the UK's capacity to build such ships - the welding alone would require a huge import of labour from Poland. The Navy in its present diminished state probably does not have the manpower to man or maintain such vessels - and by all accounts underestimated the crew levels in the original prospectus. Instead of the projected 1,500, they will need at least 3,000.
Aircraft carriers are designed round the aircraft, not the other way round. The aircraft in the Navy carrier design is the US Lockheed Martin F-35 Joint Strike Fighter, currently the most expensive defence procurement item in history. Current US costings are around $250bn and rising - even the US Navy and Air Force are cutting back severely on the number they want. The UK is down for some 140, plus the 232 Typhoons projected for the RAF. There are crews for only about 90 combat aircraft in the British services.
Meanwhile troops are suffering from gaps left by past efforts to second-guess the future. There are not enough medium-lift helicopters, and they and their crews cannot be produced overnight or over three months, apparently. Above all there is not enough combat power in Afghanistan, where the British force can still barely muster 1,000 fighting soldiers at most for operations.
But there is one expense on which Mr Brown knows he cannot scrimp - soldiers' pay and conditions. Young men and women should not be asked to serve, and if necessary risk dying, at little above the minimum wage.
The kit we can ditch
Today's armed forces were shaped by a review in 1998 that foresaw expeditionary forces sent to enforce an ethical foreign policy. But analysts worry that the commitments made to "big ticket" items, such as two new aircraft carriers with JSF fighters, are not worth the price. And previous procurement horrors such as three Astute submarines (already £1bn over budget) could be a tempting target for a Chancellor looking to make sums add up. It is thought Britain has ordered too many weapons systems and not enough intelligence capability to ensure the right targets are identified and hit.
The kit we must have
In the short term, military airlift, particularly using helicopters, is desperately needed. The shortage of trained crews to fly what craft exist is one of the clearest signs of the manpower gaps opening up after years of overstretch. To attract and retain the tens of thousands of specialists modern armed forces need will cost billions in increased funding over the coming years. An emerging 'radical' group of military thinkers believes it is time Britain significantly beefed up its number of soldiers to counter emerging threats. It is "boots on the ground", they say, not hi-tech kit, that will help keep Britain safe in the 21st century.Reuse content