The national security strategy to be published shortly will be the first vital attempt to define the threats - military, political and existential - to this country's security, values and way of life since the 9/11 terrorist atrocities almost seven years ago. It goes to the heart of the Government's strategic priorities in a world dominated by terrorism, instability and global change. It will focus public attention on Britain's defence, intelligence services, resources and preparedness and is intended not only to underline their continued importance but to convince an often complacent public that confronting these new challenges demands money, commitment and public support.

It is high time the challenges were spelt out. The most visible is terrorism, the cancer still spreading through the body politic. Over the years, Britons have become wearily familiar with the consequences: the airport queues, passes and identity checks, and ubiquitous surveillance. Less visible is the degree to which Government has had to respond. Almost all decisions now include a security dimension. The Prime Minister is in almost daily consultation with intelligence officials. Every four to six weeks there is a meeting of the newly constituted ministerial committee on national security, international relations and development. The security services have had a vast increase in staff and workload: half the global operations of MI6 now target terrorism.

At the same time, Britain has seen an unprecedented peacetime increase in military commitments. The wars in Iraq and Afghanistan as well as peacekeeping in the Balkans have stretched resources to breaking point, undermining morale and capabilities. The Government's failure to fund continuing operations properly has exacerbated arguments over force strength, procurement and multimillion-pound decisions on carriers, aircraft and weapons systems. Delay is costly.

A national security strategy must look at these demands and gauge the necessary response. The document will inform much needed debate on Britain's inadequate level of military spending, staffing and equipment. It must give essential factual underpinning to hard decisions and make the arguments needed to win public backing. Voters need to know why failure in Afghanistan would pose a long-term security threat to this country; why MI6 runs agents in Pakistan; and why MI5 needs to train more counter-intelligence officers. Unfortunately, the document is said to be getting ever wider and woollier in its definitions of threats to national security. International crime, drug rings, migration and climate change are also going to be identified as long-term threats. The globalisation of criminal cartels threatens the rule of law. Changes in rainfall or crop failures across the globe would exacerbate poverty and migration and could provoke war. In the formulation of defence policy, those issues are important but secondary. While a holistic approach to Britain's future is sensible, our national security strategy needs to have a focus. It needs to be clear that defence is different from development. Definition creep begets mission creep. It is not in the interests of either military strategy or the aid agenda to confuse the two. To an overstretched and underfunded military the decision to combine hard threats with soft ones will smack of political squeamishness.

As Chancellor, Gordon Brown took a parsimonious and unsympathetic approach to security issues. As Prime Minister, his overriding duty is to protect this country from harm.