The Pentagon revealed yesterday it suffered one of its largest losses ever of sensitive data this spring in a cyberattack by a foreign government. It is a dramatic example of why the military is pursuing a new strategy emphasizing deeper defenses of its computer networks, collaboration with private industry, and new steps to stop "malicious insiders."

William Lynn, the deputy secretary of defense, said in a speech outlining the strategy that 24,000 files containing Pentagon data were stolen from a defense 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 did not say which nation.

"We have a pretty good idea" who did it, Lynn said 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 penetrate US computer networks to steal data and to attack them in ways that damage US defenses or cause deaths.