But the cutting edge of cyberwar is in the invasion of computer systems to manipulate the machinery that keeps the country going — exactly what the United States was doing to those Iranian centrifuges as it ran Olympic Games. “Somebody has crossed the Rubicon,” Gen. Michael V. Hayden, the former director of the C.I.A., said in describing the success of the cyberattacks on Iran. General Hayden was careful not to say what role the United States played, but he added: “We’ve got a legion on the other side of the river now. I don’t want to pretend it’s the same effect, but in one sense at least, it’s August 1945,” the month that the world first saw the capabilities of a new weapon, dropped over Hiroshima and Nagasaki.

That was deliberate overstatement, of course: the United States crashed a few hundred centrifuges at Natanz, it did not vaporize the place. But his point that we are entering a new era in cyberattacks is one the administration itself is trying to make as it ramps up American defenses. Defense Secretary Leon E. Panetta — a key player in the Iran attacks — warned last year that the “next Pearl Harbor we confront could very well be a cyberattack that cripples our power systems, our grid, our security systems, our financial systems.”

IN March the White House invited all the members of the Senate to a classified simulation on Capitol Hill demonstrating what might happen if a dedicated hacker — or an enemy state — decided to turn off the lights in New York City. In the simulation, a worker for the power company clicked on what he thought was an e-mail from a friend; that “spear phishing” attack started a cascade of calamities in which the cyberinvader made his way into the computer systems that run New York’s electric grid. The city was plunged into darkness; no one could find the problem, much less fix it. Chaos, and deaths, followed.

The administration ran the demonstration — which was far more watered-down than the Pentagon’s own cyberwar games — to press Congress to pass a bill that would allow a degree of federal control over protecting the computer networks that run America’s most vulnerable infrastructure. The real lesson of the simulation was never discussed: cyberoffense has outpaced the search for a deterrent, something roughly equivalent to the cold-war-era concept of mutually assured destruction. There was something simple to that concept: If you take out New York, I take out Moscow.

But there is nothing so simple about cyberattacks. Usually it is unclear where they come from. That makes deterrence extraordinarily difficult. Moreover, a good deterrence “has to be credible,” said Joseph S. Nye, the Harvard strategist who has written the deepest analysis yet of what lessons from the atomic age apply to cyberwar. “If an attack from China gets inside the American government’s computer systems, we’re not likely to turn off the lights in Beijing.” Professor Nye calls for creating “a high cost” for an attacker, perhaps by naming and shaming.

Deterrence may also depend on how America chooses to use its cyberweapons in the future. Will it be more like the Predator, a tool the president has embraced? That would send a clear warning that the United States was ready and willing to act. But as President Obama warned his own aides during the secret debates over Olympic Games, it also invites retaliatory strikes, with cyberweapons that are already proliferating. In fact, one country recently announced that it was creating a new elite “Cybercorps” as part of its military. The announcement came from Tehran.