The White House sent a secret “hotline”-style message to Russia on Oct. 31 to warn against any further cyber-meddling in the U.S. election process. Russia didn’t escalate its tactics as Election Day approached, but U.S. officials aren’t ready to say deterrence worked.

The previously undisclosed message was part of the high-stakes game of cyber-brinkmanship that has been going on this year between Moscow and Washington. How to stabilize this relationship without appearing to capitulate to Russian pressure tactics is among the biggest challenges facing President-elect Donald Trump.

The message was sent on a special channel created in 2013 as part of the Nuclear Risk Reduction Center, using a template designed for crisis communication. “It was a very clear statement to the Russians and asked them to stop their activity,” a senior administration official said, adding: “The fact that we used this channel was part of the messaging.”

According to several other high-level sources, President Obama also personally contacted Russian President Vladimir Putin last month to caution him about the disruptive cyberattacks. The senior administration official wouldn’t comment on these reports.

The private warnings followed a public statement Oct. 7 by Director of National Intelligence James Clapper and Secretary of Homeland Security Jeh Johnson charging that “Russia’s senior-most officials” had authorized cyberattacks that were “intended to interfere with the U.S. election process.”

The senior administration official said Russia gave a “noncommittal” response to the Oct. 31 message, neither acknowledging the U.S. charges nor denying them. But the official confirmed reports by other high-level sources that after the public and private warnings, Russia did not increase its cyber-activity and may have reduced it.

“We did not see an escalation of Russian cyber-activity aimed at either trying to disrupt the election process or trying to influence the process, in the month leading up to the election,” said one senior official. A second senior official cautioned, however, that it was too early to say “whether the Russians were deterred” from additional activity.

The White House feared a last-minute Russian cyber-onslaught right up to Nov. 8, but it apparently never came. “We saw no evidence of any systematic attempt to disrupt the election on Election Day,” the first official said.

These disclosures about secret U.S.-Russia contacts are the latest chapter in the story of heightened confrontation between the two countries — a process that Putin and Trump say they are seeking to reverse. Putin phoned Trump on Monday to discuss ways to improve current “unsatisfactory” relations after Trump takes office and seek a “partner-like dialogue,” according to a Kremlin statement.

The Obama administration has grappled with how to establish norms of deterrence in cyberspace that check destabilizing actions by an aggressive, risk-taking Russia. The White House thought it was making progress with a joint statement at the November 2015 G-20 summit in Antalya, Turkey, which affirmed that “international law applies to state behavior in cyberspace.” The United States argues that this commitment includes observing laws of armed conflict that require proportionality and limited collateral impact in whatever battlespace. But the Obama administration fears Russia is ignoring these limits.

The Obama administration is ready to explore these issues further with Russia through a little-known “working group” created under a defunct “presidential bilateral commission.” The working group last met in April in Geneva. At that meeting, according to the White House, “both sides discussed the possibility of expanding the quantity and scope of information sharing about malicious activity occurring on the networks of both countries.”

Those words ring hollow now, in light of alleged Russian activities this year.

Russia experts in the Obama administration caution their successors: “It will be very difficult for the next administration . . . to know what Russia’s intentions are and whether you can have confidence that they will live up to their commitments,” said the second official. Russia has shown “increasing willingness to take risky actions,” and “old assumptions about the careful, calculating, risk-averse nature of Russian leadership . . . seem to be shifting.”

Republican Sen. John McCain (Ariz.), a longtime critic of Russia, issued a similar warning Tuesday about Putin’s professed desire for “partner-like” relations. “We should place as much faith in such statements as any other made by a former KGB agent who has plunged his country into tyranny, murdered his political opponents, invaded his neighbors, threatened America’s allies and attempted to undermine America’s elections,” he said.

A new Cold War has begun in cyberspace. Trump seems to want detente. But first he should think carefully about how to establish clear norms of deterrence in this new domain.

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