“IT’S AN honor to be with you,” President Trump told Russian President Vladi­mir Putin Friday as they met for the first time. No. Wrong. It is not an honor to sit down with the leader of a regime that invades peaceable neighbors, covertly interferes in the elections of democratic nations, and orchestrates and tolerates the assassination of domestic political opponents and journalists.

That doesn’t mean it’s wrong to meet. As Secretary of State Rex Tillerson said after the leaders ended two-plus hours of conversation, it is crucial for the world’s two major nuclear powers to engage with each other, as they did during the Cold War. “How do we live with one another? How do we work with one another?” Mr. Tillerson said. He said the two leaders spent much time on two specific areas where, at least in theory, cooperation could be mutually beneficial: the war in Syria and non-interference in future U.S. elections.

Whether cooperation with the Putin regime is possible in practice, in those or other areas, is less clear.

On the elections, Mr. Trump entered the talks in a weak position of his own devising. In a news conference Thursday, the U.S. president once again cast doubt on the fact of Russian meddling in the 2016 election, which U.S. intelligence agencies, and other politicians of both parties who have seen the evidence, do not doubt. He muddied the waters further on Friday morning with a bizarre tweet about John Podesta, campaign chairman for Mr. Trump’s 2016 Democratic presidential rival, Hillary Clinton. According to Mr. Tillerson, the U.S. president then raised with Mr. Putin “the concerns of the American people” about Russia’s interference; let’s hope he made clear that they are his concerns as well, and that there will be consequences for that meddling.

It would be useful if the two nations could begin work on cyberwar rules of the road, as they once talked about ways to lessen the danger of nuclear conflict. (In fact, it would be useful if they resumed talking about nuclear weapons, too.) Such rules could include pledges of non-interference in future elections and ways to test those pledges. But such forward-looking endeavors, which Mr. Tillerson emphasized, can’t override the ongoing investigations by Congress and the FBI into Russia’s past actions and the importance of making clear how unacceptable the interference was.

It would be positive, too, if the two countries could work together toward a peaceful future for the ravaged nation of Syria. Mr. Tillerson said the two leaders discussed a “de-escalation agreement,” with Jordan also participating, for a region in Syria’s southwest. He held out hope for larger-scale cooperation. “By and large, our objectives are exactly the same,” Mr. Tillerson asserted. If that’s so, one nation or the other has changed its objectives. Mr. Putin has long been interested in shoring up the murderous regime of Bashar al-Assad and has worked with Iran to do so; the U.S. position has been that lasting peace will be impossible as long as Mr. Assad is in power. Although neither President Barack Obama nor the current administration was willing to back up that assessment with sufficient aid to Mr. Assad’s opponents, the assessment was, and remains, accurate.