What’s worse: If Trump and Putin get along, or if they don’t?

It seems unlikely, given the American uproar about Russia’s interference in the 2016 election, and signs in the Russian media that Vladimir Putin is souring on Donald Trump, that the two presidents will make any major agreements when they meet Friday on the sidelines of the G20 summit in Hamburg.

What is most likely to emerge is a sense of how much, if any, of their fall bromance remains. But that raises key questions: What determines whether personal relations between presidents are good or bad? How important are the effects of good or bad chemistry? The answer, past summits suggest, is that coziness with Russian leaders is not necessarily nefarious and that it can help to keep the peace with Moscow, but good personal chemistry is hard to come by and bad personal relations can be positively dangerous. In the case of Trump and Putin, both good and bad chemistry could be dangerous.

The case of Franklin Delano Roosevelt and Josef Stalin demonstrates the dangers of trying to cultivate good relations with a paranoid Russian leader. To Roosevelt, Stalin resembled a no-nonsense American politician with whom he thought he could do business. He tried to win over the Soviet leader at the 1943 Tehran conference by teasing British Prime Minister Winston Churchill, and the more Churchill scowled, FDR later told his Labor Secretary Frances Perkins, “the more Stalin smiled.” After “Stalin broke into a deep, hearty guffaw,” Roosevelt continued, “I kept it up until Stalin was laughing with me and it was then that I called him ‘Uncle Joe.’” After that, “our relations were personal.” The “ice was broken and we talked like men and brothers.”

In the case of Trump and Putin, both good and bad chemistry could be dangerous.

In fact, Stalin saw through Roosevelt’s charm and took it to be trickery, which only made him more, not less, suspicious of FDR. Churchill, Stalin later told the Yugoslav politician Milovan Djilas, “is the kind who if you don’t watch him, will slip a kopeck out of your pocket! And Roosevelt? Roosevelt is not like that. He dips in his hand only for bigger coins.Despite this mistrust, the two leaders were able to hold the Grand Alliance together until the end of the war. But their agreement on the outlines of the postwar order contained the seeds of the Cold War.

The case of John F. Kennedy and Nikita Khrushchev suggests what can happen when personal chemistry is bad. Kennedy and Khrushchev were as different personally as their countries were opposed politically at the height of the Cold War. Kennedy — young, rich and highly educated — constituted a personal challenge to the ill-educated, uncultured, former peasant who now led the USSR. But in Khrushchev, Kennedy was facing a rough, tough older man who in some ways resembled the father who had dominated his childhood. Ignoring the advice of his chief advisers on the USSR, Kennedy tried at the June 1961 Vienna summit to engage Khrushchev in a candid but calm exchange about their ideological differences. But when Khrushchev, who was as explosive as he was impulsive, erupted in a defensive harangue he intimidated his young counterpart. “Roughest thing in my life,” JFK told New York Times columnist James Reston afterward. Khrushchev’s impression of Kennedy: “This man is very inexperienced, even immature.” The encounter encouraged Khrushchev to send Soviet missiles to Cuba, bringing the world closer than it has ever been to nuclear war.

A demonstrator holds up a sign of Vladimir Putin during an anti-Trump “March for Truth” rally | Eduardo Munoz Alvarez/Getty Images

Boris Yeltsin resembled Khrushchev: boisterous, boastful, blowing hot and cold at his many summits with Bill Clinton. But thanks partly to the personal chemistry between them, Clinton reacted to Yeltsin’s fireworks much more calmly than Kennedy had to Khrushchev’s. Clinton had high hopes for Russia and for Russian-American relations following the collapse of the Soviet Union. He hoped to see Russia adopt Western-style political and economic institutions while becoming a genuine ally of the West. In theory, Yeltsin was open to both these outcomes, but in practice, as Russia suffered through terrible troubles at home, including an economic crash combined with raging inflation and an explosion of corruption, he chafed at the arrogance of American advisers promoting democratic capitalism while Washington helped to expand NATO and tried to dictate the outcomes of wars in Bosnia and Kosovo.

The fact that a weakened Russia was more a supplicant than a threat to the United States helped keep relations on track. But so did the way Clinton took Yeltsin’s explosions in stride. Clinton saw in Yeltsin an “ol’ boy” like himself, referring to him in conversations with advisers as “ol’ Boris.” That helped him to roll with Yeltsin’s punches and laugh off Yeltsin’s excesses, in which Clinton seemed to recognize parallels to his own gigantic appetites. But in retrospect, Clinton underestimated the depths of Moscow’s brewing resentment.

On the surface, the case of Ronald Reagan and Mikhail Gorbachev looks most like that of Trump and Putin. When Gorbachev became Soviet leader in March 1985, the two countries seemed to be, as they do now, in a new Cold War. And the Geneva summit that November produced little or nothing in the way of concrete agreements. But both leaders regarded the personal chemistry they achieved at Geneva as a breakthrough that nearly led a year later to an agreement to eliminate nuclear weapons. In June 1988, when Gorbachev’s transformation of Soviet domestic politics and foreign policy was accelerating, Reagan declared in during the Moscow summit that the USSR was no longer the “evil empire” it had been in “another time, another era.”

The fact that Reagan and Gorbachev had so much in common helped to produce the personal bond between them. Both came from “small farming communities,” Reagan said afterward, and yet here they were at Geneva “with the fate of the world in their hands.” Both had mostly happy childhoods in harsh times. But the content of what they had in common was particularly important: both were innately optimistic, convinced that people were basically good and could be trusted to do what was right.

A picture shows a souvenir kiosk offering a drawing depicting Russian President Vladimir Putin holding a baby with the face of U.S. President Donald Trump, based upon a propaganda poster showing late soviet leader Joseph Stalin holding a baby | Mladen Antonov/AFP via Getty Images

Trump and Putin have much in common, too, beyond their obvious authoritarian bent. Both came to power without the support of a political party establishment. Both seem to understand capitalism (in words used by Russia analysts Fiona Hill and Clifford Gaddy about Putin) as “wheeling and dealing,” as “about finding and using loopholes in the law, or creating loopholes.” Both rely disproportionately on small inner circles of close advisers. Both use profanity to mobilize the masses. These similarities help to account for their mutual attraction.

But other parallels portend mutual enmity and may have already begun to drive them apart. In contrast to Reagan and Gorbachev, Trump and Putin both believe in what the New Yorker writer Ken Auletta once summarized as the philosophy of Trump’s mentor, lawyer Roy Cohn: “Everyone lies, smears, covers up, protects their friends. The rules of the game don’t count as much as winning.” And Putin’s grade school teacher’s description of him seems to fit Trump, as well: “Volodya never forgives people who betray him or are mean to him.”

If Trump and Putin get along too well in Hamburg, that could come at the expense of American allies in Ukraine and Syria. If they don’t, and if each man blames the other for their split, their tendency to lash out at those who betray them could eventually lead to a dangerous confrontation.

William Taubman, professor emeritus of political science at Amherst College, is the author of “Stalin’s American Policy, Khrushchev: The Man and His Era,” and “Gorbachev: His Life And Times,” to be published this September.