Ministers are to begin the process of establishing the UK's future defence strategy and what resources will be required.
A green paper published later today will lay the ground for the first full strategic defence review in twelve years, to take place after the general election.
None of the three main parties have promised to protect the defence budget from future spending cuts, although Gordon Brown says that Afghanistan's budget will be ring-fenced.
Today's green paper is expected to look at issues such as Britain's future global role, the capability of the armed forces, and how they should be equipped. It will draw upon the campaigns in Afghanistan and Iraq.
Last summer's updated security strategy said that the UK did not face a direct threat from any single country, but a "diverse and interconnected set of threats".
Ministers are likely to focus on how the UK can adapt to these challenges while the budget is under intense pressure.
