Cyberwarfare was discussed quite openly in the 1990s, though technological capabilities and targets were far more limited than they are today, said Jason Healey, who heads the Cyber Statecraft Initiative at the Atlantic Council in Washington.

“Our current silence dates back 8 or 10 years, and N.S.A. is a big reason,” said Mr. Healey, who is working on a history of cyberwarfare.

The National Security Agency, which plays a central role in Cyber Command, traditionally breaks foreign codes and eavesdrops on foreign communications; it is among the most secretive agencies in government. Years ago it pioneered the field of cyberespionage: breaking into foreign computer systems in order to collect intelligence. The same skills and reflexive secrecy of spies carried over to cyberwarfare, Mr. Healey said. American officials have long preferred to talk cyberdefense, leaving the attack side in the shadows.

The increased candor recently about cyberoffense results not from a policy change, officials say, but from an inevitable acceptance of attacks on computer networks as a standard part of military and intelligence capabilities. The fact that dozens of Beltway contractors see cyberwarfare as one of the few parts of the defense budget that are likely to grow is also a factor.

When Darpa announced a “proposers’ day workshop” for its Plan X program, the “overwhelming response from industry and academia” led the defense research agency to expand the event to an extra day, the agency said in a statement. (A Darpa spokesman declined to comment further on Plan X.)

Just as drone-fired missiles have never been a secret to those on the ground, so cyberattacks have consequences that cannot be hidden, even if their origin may be initially uncertain. The computer worm called Stuxnet, devised by the United States and Israel to destroy Iran’s nuclear centrifuges, was quickly detected by computer security experts when it infected networks around the world in 2010 — but remains highly classified.

Hence the Cyber Command legal conference, which avoided specific cases while dwelling on principles. Mr. Koh, of the State Department, told the conference that the United States carries out “at least two stages of legal review” on cyberwarfare operations — considering whether the law of war prohibits the use of “new weapons” altogether and, if not, how the law governs their use in “each particular operation.”