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After the launch of the strategy, former senior Homeland Security Department and National Security Agency official Stewart Baker told The Washington Times it was “not completely comforting.”

“It’s like hearing that our nuclear-war strategy is to build more fallout shelters,” he said. Current defenses, “even the ones we hope to have tomorrow, will not deter adversaries or deny them the benefits of an attack,” Mr. Baker said.

Mr. Lynn acknowledged that offensive cybertools were outpacing the best defenses, but said only nation states have the most effective weapons. Although it is often difficult to determine with certainty where cyber-attacks originate, he said, they could be deterred.

“U.S. military power offers a strong deterrent against overtly destructive attacks,” he said, “Although attribution in cyberspace can be difficult, the risk of discovery and response for a major nation is still too great to risk launching destructive attacks against the United States.”

Terrorists and rogue nations that could not be deterred did not, at the moment, have the capability to launch such massively destructive attacks, he said. “There will eventually be a marriage of capability and intent,” and the country had “a window of opportunity — of uncertain length” in which to strengthen the nation’s cyberdefenses, he added.