The spies had come without warning. They plied their craft silently, stealing secrets from the world's most powerful military.They were at work months before anyone noticed their presence. And when American officials finally detected the thieves, they saw it was too late.The damage was done . . . It could have been the plot of a Tom Clancy thriller: Chinese hackers break into American defence contractors and steal the plans for a new multi-billion dollar fighter jet.In fact, it is just one dispatch from the frontline of a new form of warfare. Our wars are increasingly being fought online. GCHQ and the NSA gather vast amounts of information from the internet - and do so with the complicity of companies like Google and Fac.The American military fields teams of hackers who can, and do, launch computer virus attacks against enemy targets. And with the majority of civil infrastructure - things like nuclear power stations, hospitals, airports and banking systems - now run across the internet, the next 9/11 could be a cyber-attack. Welcome to the modern world of warfare.