Q&A: strategic defence and security review
As the government prepares to publish the country's first large-scale strategic defence and security review since 1998, all branches of the UK's armed forces will need to be ready to make sacrifices.
Q: What is the strategic defence and security review?
The strategic defence and security review - or SDSR - will decide the future shape and size of Britain's armed forces.
It is meant to be driven by the government's strategic vision of Britain's place in the world, addressing the threats and risks the country is likely to face in the future.
The SDSR will be released in two separate documents.
On Monday the government will publish a broader paper that explains the strategic thinking behind the decisions being taken.
On Tuesday, in the House of Commons, David Cameron will give the details of the defence review.
Q: Why is it taking place?
The last defence review took place more than a decade ago.
Since then we have had the attacks on 9/11 and the wars in Iraq and Afghanistan.
None of these were predicted in the last defence review. So a major strategic rethink is long overdue.
But the government's been accused of rushing the process. The last defence review in 1998 took more than a year to complete. This time it is being carried out within just five months.
The backdrop is the pressure to make huge financial savings - which raises the question as to whether the Treasury is driving the SDSR rather than real strategic thinking.
In a letter leaked last month to the Daily Telegraph, Defence Secretary Liam Fox warned the prime minister that "the process is looking less and less defensible as a proper SDSR" and looking more like a "super CSR" - the Comprehensive Spending Review.
He went on to warn that the SDSR was in danger of lacking "coherence".
Q: How have the decisions been taken? Has there been agreement within the forces?
The decisions have been taken by the National Security Council. It is chaired by the prime minister, with senior ministers and the chief of the defence staff, Air Chief Marshall Sir Jock Stirrup.
Sir Peter Ricketts, the prime minister's national security advisor, is the civil servant overseeing the process.
The three heads of the armed forces have also been present for some of the meetings. But ultimately this will be David Cameron's decision.
Alongside the National Security Council discussions there have been separate talks within the Ministry of Defence (MoD) itself, involving the heads of the three armed services - the Royal Navy, the Army and the RAF.
Each service has had to put forward a range of options for cuts. The Army has agreed to sacrifice much of its heavy artillery and tanks - largely seen as weapons of the Cold War.
The RAF has reluctantly put forward options to cut its fleet of fast jets - Tornadoes or Harriers - in the hope that they will get the new Joint Strike Fighter instead. The Royal Navy, which wants two new aircraft carriers at a cost of more than £5bn, is expected to lose some of its surface fleet.
Q: What level of cuts can the armed forces expect?
We now know that, following the intervention of the prime minister, the MoD will face a cut of around 8% in its budget.
This will be interpreted by many as a "moral victory" for Liam Fox who had been under huge pressure to make even bigger savings. The MoD had been asked by the Treasury to make cuts of between 10% and 20%.
Even though this was less than most government departments, remember that the MoD already has a black hole in its budget of £38bn left by the last Labour government. Filling that void would, in itself, translate to a cut of around 10% over the next four years.
So another 10% on top of that would have had a massive impact on the capabilities and the size of the armed forces. Cuts of around 8% will still be painful. The former First Sea Lord, Admiral Lord West, says it would be wrong to think that defence has got off lightly.
Q: Are any areas protected? Are some more vulnerable than others?
The only military hardware safe from the SDSR is the fleet of four Vanguard submarines that carry Britain's trident nuclear deterrent.
The Conservatives promised in their manifesto to retain Britain's continuous at-sea deterrence (CASD).
The Tories are also committed to replacing that fleet when it comes to the end of its life in the middle of the next decade.
However, the issue has already created tensions within the coalition government, with the Liberal Democrats arguing for a cheaper alternative to Trident.
Added to that, Liam Fox has been locked in a battle with the chancellor George Osborne over who should pay the £20bn it would cost to replace the Trident fleet.
For now, the debate has been put on the back burner - with a final decision on Trident's replacement to be delayed until after the next election.
Apart from operations in Afghanistan, nothing else is protected from the cuts.
However, the government has dismissed suggestions that one of the three armed services could be scrapped to make the necessary savings.
So the RAF, navy and Army will survive, though all three are likely to be smaller.
Q: Will the strength of the armed forces and its operations be affected? What about Afghanistan?
The strength of the armed forces will inevitably be altered by the SDSR, though the government insists that Britain's commitment to the war in Afghanistan will not be affected.
The head of the Army - and soon to be Chief of the Defence Staff - Gen Sir David Richards, has mounted a vigorous campaign to avoid large cuts in troop numbers.
Initial reports suggested the Army could lose as many as 20,000 men. But Gen Richards's skilful lobbying appears to have staved off the worst - at least until 2015 - the date by which David Cameron says British combat troops will leave Afghanistan.
In theory the costs of the Afghan war will continue to come out of the Treasury's Special Reserve fund.
In practice, the war still swallows up much of the MoD's resources. But whoever pays, the government remains committed to the war in Afghanistan.
Q: How will the security services be affected by this review?
The security services are unlikely to lose out on the scale of the armed services.
Counter terrorism operations - which have already received huge injections of cash - will remain a priority.
Whitehall insiders say that in the national security strategy document counter-terrorism is an area considered so vital there are no plans for any significant cuts to the intelligence agencies.
Featuring for the first time is cyber security, with an emphasis on the need to protect Britain's critical national infrastructure from malicious hacking and on preventing the individual from the growing menace of online cyber crime.
And then there is counter-proliferation, the need to prevent weapons of mass destruction falling into the hands of countries or non-state organisations perceived to be hostile to Britain.