To be effective on the battlefield, tactical weapons need to be widely dispersed and available for immediate use, making them more vulnerable to theft, sabotage, and unauthorized use. They may also make nuclear war more likely. Because the destructive effects of tactical weapons are smaller, the temptation to use them may be greater. Once the “nuclear taboo” has been broken, nobody can be certain what will happen next. At Hiroshima and Nagasaki, nuclear weapons were used against a nation that didn’t have them.

Russia and the United States possess about ninety per cent of the world’s approximately fifteen thousand nuclear weapons, maintaining arsenals large and diverse enough to hit a variety of targets. The most recent Nuclear Employment Strategy of the United States, issued by the Obama Administration, in 2016, is a veritable jobs program for weapons of mass destruction. It emphasizes the importance of destroying counterforce (military) targets rather than countervalue (civilian) targets, and it vows to “minimize collateral damage to civilian populations,” in keeping with international law. The Trump Administration’s Nuclear Posture Review advocates a strategy that sounds oddly elegant: “tailored deterrence.” Its objectives include preventing a nuclear attack on the United States, protecting American allies from attack, and, if deterrence fails, ending “any conflict at the lowest level of damage possible and on the best achievable terms.”

Russia has also changed its nuclear strategy. During the Cold War, the Soviet Union claimed that it would never be the first to use nuclear weapons. But Russia is no longer confident that its conventional forces are superior to those of NATO, and so it has embraced an “escalate to de-escalate” strategy, raising the possibility of the use of tactical weapons against NATO troops. The strategy is based on a faith that low-yield nuclear blasts will impose “tailored” damage on NATO, de-escalate the conflict, and force a ceasefire. The strategy presumes that NATO won’t retaliate by using nuclear weapons, too. The change in Russian doctrine has prompted the Trump Administration to seek new low-yield, tactical weapons. The Administration believes that its new tactical weapons will deter the Russians from ever using their own—reversing a bipartisan consensus that for the past quarter century has regarded these weapons as gravely and needlessly dangerous.

At the height of the Cold War, the United States kept about seven thousand tactical nuclear weapons in Europe. The utility of those weapons was always in doubt. During Carte Blanche, a war game conducted in 1955, three hundred and thirty-five NATO tactical weapons were used against invading Soviet tanks and troops, for the most part on battlefields in Germany. Robert McNamara later outlined the results: “It was estimated that between 1.5 and 1.7 million people would die and another 3.5 million would be wounded—more than five times the German civilian casualties in World War II—in the first two days.” Those estimates did not include deaths from illness, radiation poisoning, or Soviet nuclear weapons. Subsequent war games confirmed the findings of Carte Blanche: if NATO ever used tactical weapons to defend Germany, it would destroy Germany. The mere existence of tactical weapons could destabilize a crisis and make it end badly. During the Cuban Missile Crisis, President John F. Kennedy and his advisers didn’t know that the Soviet forces on the island and in the sea surrounding it not only had tactical weapons but also had the ability to use them without consulting Moscow. An American attack—contemplated for days at the White House and nearly set in motion—would have unwittingly led to a nuclear war.

After the collapse of the Soviet Union, in 1991, the United States unilaterally removed all of its tactical weapons from South Korea and almost all of them from Europe. The Chairman of the Joint Chiefs of Staff at the time, General Colin Powell, had trained in the employment of tactical nuclear weapons as a young officer and thought that they “had no place on a battlefield.” With the support of every member of the Joint Chiefs of Staff, Powell persuaded Secretary of Defense Dick Cheney and President George H. W. Bush to get rid of them, and over the years the size of NATO’s tactical nuclear stockpile fell by ninety-seven per cent.

Today, the United States keeps about two hundred tactical weapons at six NATO bases in Germany, Belgium, Turkey, Italy, and the Netherlands. The weapons are B-61 bombs designed to be carried by fighter planes. They have no assigned role in NATO’s war plans, and their military usefulness is “practically nil,” according to General James Cartwright, a former commander of the United States Strategic Command. The B-61 bombs have been retained as symbols of America’s commitment to the defense of NATO, despite concern that the weapons are vulnerable to theft by terrorists, sabotage, and attack, especially in Turkey. A few B-61s could fit in the bed of a pickup truck.

Experts fear that B-61 nuclear bombs, which can fit in the bed of a pickup truck, are vulnerable to theft from U.S. bases abroad. A retired Marine general described their military usefulness as “practically nil.” Photograph Courtesy Randy Montoya / Sandia National Laboratories

The Trump Administration is moving forward with plans to modernize the B-61 and would like to mount low-yield tactical warheads on submarine-based missiles. The advantage of basing tactical weapons on a submarine is that they will be hidden underwater—and therefore will be less likely to be stolen, attacked, or become the subjects of political protests. The disadvantage is that Russia will have no way of knowing whether a missile launched from a submarine is carrying a tactical warhead meant to destroy a tank battalion on the battlefield or a strategic warhead fired to destroy an underground leadership bunker in Moscow.

The glaring problem of how the President of the United States and the President of Russia might reliably communicate and negotiate during a limited nuclear war has never been resolved. The Moscow-Washington Direct Communications Link, known as the “hotline,” isn’t a voice link with matching red telephones, as portrayed in Hollywood thrillers. It’s a dedicated computer link that transmits encrypted e-mails between the Kremlin and the Pentagon. A recent photograph of the hotline is not reassuring: it looks like a computer terminal you might find in the business center at a Marriott hotel.

The return of tactical weapons is the most controversial aspect of Trump’s Nuclear Posture Review. The new policy assumes that American tactical weapons will deter the use of Russian tactical weapons, raising “the nuclear threshold” and making “nuclear employment less likely.” Sam Nunn, a former chairman of the U.S. Senate Committee on Armed Services and a co-founder of the Nuclear Threat Initiative, has argued against that sort of thinking for more than forty years. He fears that the chance of accidents, miscalculations, and blunders with tactical weapons—as well as the pressure to “use them or lose them” in battle—greatly increase the risk of an all-out nuclear war. Like so many of the disagreements about nuclear strategy, this one cannot be settled with empirical evidence, and selecting the wrong policy could be catastrophic. As Nunn observed in 1974, after a tour of NATO’s tactical nuclear units, “Nobody has any experience in fighting nuclear wars, and nobody knows what would happen if one were to start.”

On the morning of August 6, 1945, Setsuko Thurlow, then thirteen years old, was preparing to decode messages on the second floor of the Army headquarters in Hiroshima. About twenty girls from her school worked beside her, and thousands of other middle schoolers were employed at patriotic tasks throughout the city as part of the Student Mobilization Program. Thurlow noticed a bright bluish-white flash outside the window at 8:15 A.M. She never saw the mushroom cloud; she was in it. She felt herself fly through the air, blacked out, and awoke pinned in the rubble of the collapsed building, unable to move. Lying there in silence and total darkness, she had a feeling of serenity. And then she heard the cries of classmates trapped nearby: “God, help me!,” “Mother, help me!” Someone touched her, removed the debris on top of her, and told her to crawl toward the light. She somehow made it out safely and realized that what was left of the headquarters was on fire. A half dozen or so other girls survived, but the rest were burned alive.