Last week we briefly rode a nuclear roller-coaster. Scary. A few words from Vladimir Putin launched Donald Trump — acting like he’s already president — into counterthreat mode. Fortunately, soothing exchanges calmed things within two days, but the episode may predict mercurial behavior when Trump does become president. 

Trouble started Thursday when Putin — reacting to candidate Trump’s vow to beef up the U.S. nuclear program — told his military commanders that he would “strengthen” Russia’s nuclear missile forces. Did this mean routine maintenance and modernization — which we do too — or reigniting an arms race?

Trump took it as the latter and angrily responded that we too were going to “expand” our nuclear arsenal. His spin doctors said he just aimed to stop nuclear proliferation, but Trump told Mika Brzezinski on Friday: “Let it be an arms race. We will outmatch them at every pass and outlast them all.” 

The two, based on mutual misperception of threat, goaded each other. If it kept going, Trump would have abandoned his pro-Russia tilt. From dismissing Russian hacking as “ridiculous,” he could have found that a congressional investigation has merit.  

By Saturday, however, the two had exchanged calming messages. Putin indicated that he did not mean to expand Russia’s nuclear arsenal, and Trump said the same. With the relationship back on track, Trump can still offer Putin a deal, like Russia keeping Crimea but leaving the rest of Ukraine while we lift sanctions. A renewed nuclear arms race would have blocked such “transactions.” 

The two suffer professional deformations that lead to misunderstanding. Trump’s flamboyant real-estate dealing has trained him to always attack, never lose and reflexively counterpunch. Putin, like all KGB officers, is trained to intimidate and break people, not negotiate win-win deals with them. Neither articulates an overall strategy or tolerates push-back from other countries. One important difference: icy Putin has total impulse control; seething Trump parades his impulses.

Last week’s incident should bring back discussion of nuclear strategy, which more than half a century ago was a major topic, expounded in books and courses. New vocabulary appeared: escalation, second-strike, countervalue and broken-back war. Chief scenario spinner was Herman Kahn of RAND, one of the models for Dr. Strangelove (the other was Henry Kissinger). Kahn’s calm calculation of how many decades it would take to recover from nuclear war scared everybody. (It was caricatured in the movie.)

Kahn-type reasoning held that we always need a few thousand more nuclear warheads lest the enemy try a “disarming first strike” that left us defenseless and devastated. To guard against that, the superpowers built ad absurdum until together they possessed some 70,000 warheads. If half were detonated, no one would be left to make post-apocalypse movies. 

Nuclear-war strategy receded into a minor topic. We supposed that we reached a mutual understanding that nukes could not and would not be used. President Reagan was eloquent on this. Even the military dislikes nukes: An expensive weapons system that sits forever idle but requires upkeep and elaborate security. Every billion spent on nukes means less for necessary military preparedness.

Agreements shrunk arsenals down to “only” 7,000 warheads each (1,550 of them deployed). Routine maintenance is part of our current program and nothing alarming. For one thing, nukes’ tritium gas (that makes them hydrogen bombs) has to be recharged every decade or two. Aside from (maybe) deterring attack, though, nukes are almost perfectly unusable, especially in today’s asymmetrical warfare. A jihadi with an AK-47 is a poor nuclear target.

Israel’s nukes did not dissuade Egypt and Syria from attacking in 1973. India and Pakistan, both nuclear powers, stumble toward war over Kashmir with little thought of nukes. To be sure, the 1961 Berlin Wall and 1962 Cuban Missile crises could have escalated to nuclear, but they didn’t. Mutual fear worked.

What seems to work is a concept Kahn fought, “finite deterrence”: you need only a hundred or so nukes to deter, not thousands. True, terrorists, who protect no homeland, could use nukes. Whoever provided them with nukes — say, Pakistan or North Korea — would face terrible wrath. And trace elements in the fallout mark who produced them.

The 1968 Nuclear Non-Proliferation Treaty slowed the spread of nuclear weapons, but another arms race would undermine NPT and encourage new entrants. Trump even suggested that Japan and South Korea acquire their own nukes. 

Only China would gain from a Russia-U.S. nuclear race. Xi Jinping must have chuckled last week: “The two macho white guys taunt each other into an escalating stalemate while we stand aside and build the world’s biggest economy and tighten possession of the South China Sea.”

Trump promised disruption and unpredictability, but that can soon turn into erratic behavior and the misreading of intentions. The danger is that Trump will tweet roller-coaster foreign policy, switching from like to dislike on alternate days. Eventually, a distemperate statement could jump the roller-coaster off its tracks. And Trump, all on his own, can launch nukes.