Since Russia invaded Ukraine on February 24, Russian President Vladimir Putin has given orders to increase the alert level of Russia’s nuclear forces and has made veiled nuclear threats. The blatant aggression against Ukraine has shocked Europe and the world. The war is a tragedy for Ukraine. It also exposes the limits of the West’s reliance on nuclear deterrence.

Deterrence refers to the idea that possessing nuclear weapons protects a nation from attack, through the threat of overwhelming retaliation. This concept is widely credited for helping prevent war between the United States and the Soviet Union during the Cold War. However, Russia’s invasion of Ukraine casts a harsh light on its downsides. Most obvious is that Putin is using nuclear deterrence not to protect Russia but rather to have his way in Ukraine. Russia’s nuclear weapons deter the West from intervening with conventional military forces to defend Ukraine. Despite scattered calls in the U.S. for the creation of a “no-fly zone” over some or all of Ukraine, the Biden administration has wisely resisted. In practice this would mean shooting down Russian planes. It could lead to World War III. On the other side of the ledger, NATO’s nuclear weapons presumably deter Russia from expanding the war to NATO countries, such as Poland, Romania or the Baltic states. Thus, the nuclear balance of terror likely deters a wider European war but leaves Ukraine to struggle on with only limited support and perhaps eventually to be swallowed. On balance, NATO states do not seem very reassured by their vaunted nuclear deterrence. They continue to worry about the (remote) possibility of a Russian conventional attack beyond Ukraine.

This is not the first time Putin has rattled the nuclear saber. He also did so in 2014 during Russia’s invasion of Crimea, when Russian leaders talked openly about putting nuclear weapons on alert. In 2015, Russia threatened Danish warships with nuclear weapons if Denmark joined NATO’s missile defense system. Putin likes to wave about his nuclear weapons as a reminder to the West (and perhaps to himself) that Russia is still a great power. In the current crisis, Putin clearly wants the US and NATO to know that if the West were to intervene with military force on behalf of Ukraine, he might reach for his so-called tactical (or “nonstrategic”) nuclear weapons.

In the world of nuclear weapons, tactical means an exceedingly large amount of explosive energy and strategic means even larger. Most nuclear weapons today are variable-yield, or “dial-a-yield,” providing a set amount of explosive energy that can range from fractions of a kiloton to multiples of a megaton. (For example, the U.S.’s newest version of its B61 nuclear bomb can release 0.3, 1.5, 10 or 50 kilotons of explosive energy. In comparison, the Hiroshima bomb was about 15 kilotons.) Russia has about 4,500 nuclear warheads in its arsenal. Of these, the ones of largest yield—the “strategic” weapons—are deployed on submarines, bombers and intercontinental ballistic missiles.

But Russia also possesses some 2,000 tactical nuclear weapons kept in storage facilities throughout the country, developed to be used against troops and installations in a small area or in a limited engagement. Such weapons can be launched on the same short-range missiles Russia is currently using to bombard Ukraine, such as its Iskander ballistic missile, which has a range of about 500 kilometers. And these are not the only tactical weapons that could be deployed; the United States has about 100 nuclear “gravity bombs” (with less sophisticated guidance) stationed around Europe.

Tactical nuclear weapons exist because each side fears it would be deterred from using its big city-razing weapons by their very destructiveness. By making nuclear weapons smaller and the targeting more precise, their use becomes more thinkable. Paradoxically, while this makes deterrence threats more credible, it also makes the arms more tempting to use first, rather than simply in retaliation.

No one should imagine, however, that it makes sense to use a tactical nuclear weapon. A thermonuclear explosion of any size possesses overwhelming destructive power. Even a “small-yield” nuclear weapon (0.3 kilotons) would produce damage far beyond that of a conventional explosive. (For a graphic depiction, the interactive site NUKEMAP, created by nuclear historian Alexander Wellerstein, allows you to simulate the effects of a nuclear explosion of any size anywhere on the planet.) It would also cause all the horrors of Hiroshima, albeit on a smaller scale. A tactical nuclear weapon would produce a fireball, shock waves, and deadly radiation that would cause long-term health damage in survivors. Radioactive fallout would contaminate air, soil, water and the food supply (Ukrainians are already familiar with this kind of outcome because of the disastrous meltdown of the Chernobyl nuclear reactor in 1986).

No one knows if using a tactical nuclear weapon would trigger full-scale nuclear war. Nevertheless, the risk of escalation is very real. Those on the receiving end of a nuclear strike are not likely to ask whether it was tactical or strategic. In testimony before the House Armed Services Committee on February 6, 2018, then–Secretary of Defense James Mattis stated “I do not think there is any such thing as a tactical nuclear weapon. Any nuclear weapon used any time is a strategic game changer.” Russian leaders have made clear that they would view any nuclear attack as the start of an all-out nuclear war.

Especially worrisome is the possibility that the war could escalate to the use of nuclear weapons. By increasing the alert level of Russian nuclear forces, Putin increases the risk of nuclear use through miscalculation or accident in the fog of war. In the worst scenario, if the war is going badly, Putin could reach for a tactical nuclear weapon out of desperation. While this is still unlikely, the risk is not zero. And increasing that risk is unacceptable. Although innumerable nuclear weapons have been tested over the years, not one has been used in warfare (or terrorism) since 1945. The 77-year-old tradition of nuclear nonuse—the nuclear taboo—is the single most important accomplishment of the nuclear age. It is a primary obligation of leaders today to make sure nuclear weapons are never used again. Putin and Russian Foreign Minister Sergey Lavrov should stop threatening nuclear weapons. Other leaders should express shock and outrage, and make it clear that nuclear threats are irresponsible and unacceptable.

Nuclear deterrence comes with tremendous risks and enormous costs. The arguments in favor of deterrence, although sometimes convincing, are not always true. We must acknowledge that nuclear deterrence could fail. That’s why, despite the trillions of dollars spent on nuclear arsenals, no one sleeps soundly under a nuclear umbrella—especially during a crisis such as Russia’s invasion of Ukraine.

This war will likely upend the European security order. It also demonstrates how little real protection nuclear weapons provide. The world would be better off without these weapons.