According to Marine Gen. James Cartwright, the Pentagon currently focuses 90 per cent of its cybersecurity effort on defence and 10 per cent on offence. He believes a better balance for the U.S. government as a whole would be 50-50. (Associated Press)

The Pentagon on Thursday revealed that in the spring it suffered one of its largest losses ever of sensitive data in a cyberattack by a foreign government.

It is a dramatic example of why the military is pursuing a new strategy emphasizing deeper defences of its computer networks, collaboration with private industry and new steps to stop "malicious insiders."

William Lynn, the deputy secretary of defence, said in a speech outlining the strategy that 24,000 files containing Pentagon data were stolen from a defence industry computer network in a single intrusion in March. He offered no details about what was taken but said the Pentagon believes the attacker was a foreign government. He didn't say which nation.

"We have a pretty good idea" who did it, Lynn said in an interview before the speech. He would not elaborate.

Many cyberattacks in the past have been blamed on China or Russia. One of the Pentagon's fears is that eventually a terrorist group, with less at stake than a foreign government, will acquire the ability to not only penetrate U.S. computer networks to steal data but to attack them in ways that damage U.S. defences or even cause deaths.

Terrorist groups, rogues states hard to deter

In his speech at the National Defence University, Lynn said that sophisticated computer capabilities reside almost exclusively in nation-states, and that U.S. military power is a strong deterrent against overtly destructive cyberattacks. Terrorist groups and rogue states, he said, are a different problem and harder to deter.

"If a terrorist group gains disruptive or destructive cybertools, we have to assume they will strike with little hesitation," he said.

The Pentagon has long worried about the vulnerability of its computer systems. The concern has grown as the military becomes more dependent not only on its own computers but also on those of its defence contractors, including providers of the fuel, electricity and other resources that keep the military operating globally.

At his Senate confirmation hearing last month, new Defence Secretary Leon Panetta cited "a strong likelihood that the next Pearl Harbor" could well be a cyberattack that cripples the U.S. power grid and financial and government systems. He said last weekend that cybersecurity will be one of the main focuses of his tenure at the Pentagon.

A Japanese surprise attack on the U.S. naval base of Pearl Harbor in Hawaii brought the United States into World War II.

"For the Department of Defence, our networks are really our lifeblood," Marine Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff, told reporters in an interview prior to Lynn's release of the new strategy.

As shown by the March attack on a defence industry computer network that contained sensitive defence data, the military's vulnerability extends beyond its own computers. In a new pilot program, the Pentagon is sharing classified threat intelligence with a handful of companies to help them identify and block malicious activity.

Lynn said intrusions in the last few years have compromised some of the Pentagon's most sensitive systems, including surveillance technologies and satellite communications systems. Penetrations of defence industry networks have targeted a wide swath of military hardware, including missile tracking systems and drone aircraft, he said.

Defensive approach inadequate:Cartwright

In Cartwright's view, a largely defensive approach to the problem is inadequate. He said the Pentagon currently is focused 90 per cent on defensive measures and 10 per cent on offence; the balance should be the reverse, he said. For the federal government as a whole, a 50-50 split would be about right, Cartwright argued.

"If it's OK to attack me and I'm not going to do anything other than improve my defences every time you attack me, it's difficult" to stop that cycle, Cartwright said. He added that a number of complex legal and cultural issues need to be sorted out before the Pentagon can devise a comprehensive offensive strategy.

Earlier this year, President Barack Obama signed executive orders that lay out how far military commanders around the globe can go in using cyberattacks and other computer-based operations against enemies and as part of routine espionage. The orders detail when the military must seek presidential approval for a specific cyberattack on an enemy, defence officials and cybersecurity experts told the AP.

The strategy unveiled by Lynn is oriented toward defensive rather than offensive measures. It calls for developing more resilient computer networks so the military can continue to operate if critical systems are breached or taken down. It also says the Pentagon must improve its workers' cyber "hygiene" to keep viruses and other intrusions at bay. And it calls for fuller collaboration with other federal agencies, companies and foreign allies.