Richard A. Clarke, the chairman of Good Harbor Security Risk Management, was special adviser to the president for cybersecurity in the George W. Bush administration. Steve Andreasen, a consultant to the Nuclear Threat Initiative, was the National Security Council’s staff director for defense policy and arms control from 1993 to 2001.

“We will reduce the role of nuclear weapons in our national security strategy and urge others to do the same.”

— Barack Obama, Prague, April 2009

President Obama is expected to unveil a new nuclear policy initiative this week in Berlin. Whether he can make good on his first-term commitments to end outdated Cold War nuclear policies may depend on a firm presidential directive to the Pentagon rejecting any new missions for nuclear weapons — in particular, their use in response to cyberattacks.

The Pentagon’s Defense Science Board concluded this year that China and Russia could develop capabilities to launch an “existential cyber attack” against the United States — that is, an attack causing sufficient damage that our government would lose control of the country. “While the manifestation of a nuclear and cyber attack are very different,” the board concluded, “in the end, the existential impact to the United States is the same.”

Because it will be impossible to fully defend our systems against existential cyberthreats, the board argued, the United States must be prepared to threaten the use of nuclear weapons to deter cyberattacks. In other words: I’ll see your cyberwar and raise you a nuclear response.

Some would argue that Obama made clear in his 2010 Nuclear Posture Reviewthat the United States has adopted the objective of making deterrence of nuclear attacks the “sole purpose” of our nuclear weapons. Well, the board effectively reviewed the fine print and concluded that the Nuclear Posture Review was “essentially silent” on the relationship between U.S. nuclear weapons and cyberthreats, so connecting the two “is not precluded in the stated policy.”

As the board noted, cyberattacks can occur very quickly and without warning, requiring rapid decision-making by those responsible for protecting our country. Integrating the nuclear threat into the equation means making clear to any potential adversary that the United States is prepared to use nuclear weapons very early in response to a major cyberattack — and is maintaining nuclear forces on “prompt launch” status to do so.

Russia and China would certainly take note — and presumably follow suit. Moreover, if the United States, Russia and China adopted policies threatening an early nuclear response to cyber­attacks, more countries would surely take the same approach.

It’s hard to see how this cyber-nuclear action-reaction dynamic would improve U.S. or global security. It’s more likely to lead to a new focus by Pentagon planners on generating an expanding list of cyber-related targets and the operational deployment of nuclear forces to strike those targets in minutes.

Against that backdrop, maintaining momentum toward reducing the role of nuclear weapons in the United States’ national security strategy (and that of other nations) — a general policy course pursued by the past five presidents — would become far more difficult. Further reductions in nuclear forces and changes in “hair-trigger” postures, designed to lessen the risk of an accidental or unauthorized nuclear launch, would also probably stall.

Fortunately, Obama has both the authority and the opportunity to make clear that he meant what he said when he laid out his nuclear policy in Prague in 2009. For decades, presidential decision directives have made clear the purpose of nuclear weapons in U.S. national security strategy and provided broad guidance for military planners who prepare the operations and targeting plans for our nuclear forces. An update to existing presidential guidance is one of the homework items tasked by the 2010 Nuclear Posture Review.

Cyberthreats are very real, and there is much we need to do to defend our military and critical civilian infrastructure against what former defense secretary Leon E. Panetta referred to as a “cyber Pearl Harbor” — including enhancing the ability to take action, when directed by the president, against those who would attack us. We also need more diplomacy such as that practiced by Obama with his Chinese counterpart, Xi Jinping, at their recent summit. Multinational cooperation centers could ultimately lead to shared approaches to cybersecurity, including agreements related to limiting cyberwar.

U.S. cyber-vulnerabilities are serious, but equating the impact of nuclear war and cyberwar to justify a new nuclear deterrence policy and excessive Cold War-era nuclear capabilities goes too far. It diminishes the unique threat of national devastation and global extinction that nuclear weapons represent, undermines the credibility of nuclear deterrence by threatening use for lesser contingencies and reduces the urgency for focused action to lessen nuclear dangers. Excessive rhetoric on the threat of cyberwar from the United States and blurring the distinction between cyber and nuclear attacks just makes progress toward cyber-peace more difficult.

With a stroke of his pen and his speech in Berlin, Obama can keep the United States from uploading the cyber-nuclear link.