By Paul Adams BBC Defence Correspondent

These are times of change, upheaval perhaps, for Britain's armed forces.

For the second time in five years, their very purpose, and along with that, their composition and equipment, are the subject of a thorough examination.

It is not being called, as 1998's document was, a Strategic Defence Review (SDR), but in some ways it is every bit as important.

Could the army suffer in manpower as well as heavy equipment?

SDR took account of the end of the Cold War. A "new chapter" was hastily put together in the wake of 11 September.

But now the full implications of the "war on terrorism" have been assessed and the Ministry of Defence says its time to "rebalance" the forces.

Not surprisingly, perhaps, this is being seen as a euphemism for cuts, and the Chief of the Defence Staff General Sir Michael Walker told an audience on Tuesday that some "tough choices" lay ahead.

"It would be quite wrong for us to retain systems, within a finite budget," he said, "which we know are no longer effective."

Cuts

Speaking in the Commons, the Defence Secretary, Geoff Hoon, made a very similar point.

"Resources must be directed at those capabilities that are best able to deliver the range of military effects required," he told MPs, "whilst dispensing with those elements that are less flexible."

After months of speculation and a fair amount of leaking, some aspects of the changes to come are now clear.

The army's balance of heavy and light forces will change. 4 Armoured Brigade, based in Germany, will be reformed as a mechanised brigade, with its 57 Challenger II tanks going into storeage, being sold or reassigned.

19 Mechanised Brigade, from Catterick, will meanwhile be turned into a new light brigade.

The UK's two existing light formations, 16 Air Assault and 3 Commando, have had to shoulder much of the burden in recent operations abroad.

By the end of the decade, the first of a new family of lighter armoured vehicles should come into service. These, eventually, will include a "mini tank", small enough to be transported by air.

With its emphasis on new technology, will the government still have the money to invest in basic manpower? Hoon's plans for forces

When it comes to navy and air force cuts, officials are reluctant to be drawn. There's little doubt that the navy will get its two new "super carriers" - vital platforms for the government's ability to wage expeditionary warfare, far from home - but it's still not clear how big they'll be.

Some of the fleet's older ships, and perhaps one or two of the planned Type 45 destroyers, will have to go.

And while officials insist that the first tranche of 55 Eurofighters for the RAF will be purchased, they say only that the second is still being negotiated.

Advanced weapons

Privately, they admit that it's now extremely unlikely that the government will ever buy 232 planes, as originally planned. They say Britain simply does not need so many and point to the fact that during the Iraq war, Saddam Hussein's air force failed to put up a single aircraft.

Advances in weaponry, including precision-guided munitions, also mean that desired effects can be achieved more efficiently.

It is not how many "platforms" you have (planes, ships, etc), it is what they can do that matters.

Officials predict frequent, small to medium scale operations, requiring agile, flexible forces and equipment.

They talk of what's called "network enabled capabilities" - computer-based technology which allows commanders to see everything that is happening, in real time, and take rapid decisions.

Under Donald Rumsfeld, the American military is undergoing a similar transformation.

Critics suggest the government's keen desire to keep pace with our most regular coalition partner threatens some of the British military's more traditional strengths.

One of those is the labour intensive business of peace keeping.

With its emphasis on new technology, will the government still have the money to invest in basic manpower?