Leading article: Vigilance is the only realistic defence in cyber war

The Ministry of Defence is recruiting hundreds of new cyber specialists, while the Pentagon has decided that an attack on US computer networks from another country may be deemed an act of war, meriting an armed response. Both moves are merely acknowledgement that the internet, once considered a background support system, is now a front line of modern conflict, both commercial and military.

Once the phenomenon was a curiosity, lending itself to a film such as the 1983 Wargames, about a teenage hacker who breaks into a Pentagon supercomputer and almost unleashes World War III. Today, the threat is perhaps less dramatic but more dangerous and complicated. Cyber attacks are pernicious, pervasive, and constant. The past 12 months have seen well-publicised assaults on Google, Sony, and Lockheed Martin, the largest US defence contractor. But a myriad others go unreported.

Cyber attacks have two main purposes: to steal and to disrupt. The former, exemplified by the Lockheed Martin incident, is but a 21st-century version of espionage, the world's second oldest profession. The second, as the Pentagon's new approach underlines, is war by other means. Imagine, for instance, the devastating impact of an invisible cyber bomb that took out the electricity grids of London or the north-east US for any length of time. The problem is how to deal with such attacks.

Retaliation, whether in kind or otherwise, is clearly one response – but against whom?. Consider the 2010 Stuxnet computer worm attack that disrupted Iran's suspected nuclear arms programme: perish the thought that the US and/or Israel had anything to do with it. The source of a cyber attack can be hard to pin down: is it a malicious hacker, a corporate rival, a terrorist group, or a national government? In other words, is it mischief, or is it war?

A better response might be reinforced international co-operation, perhaps an international convention. But given the nature of the cyber-beast, that is akin to trying to corral the wind. The only realistic defence is constant vigilance. Cyber wars are likely to become even more pernicious and pervasive in the future.