Tactical nuclear weapons have burst onto the international stage as Russian President Vladimir Putin, facing battlefield losses in eastern Ukraine, has threatened that Russia will “make use of all weapon systems available to us” if Russia’s territorial integrity is threatened. Putin has characterized the war in Ukraine as an existential battle against the West, which he said wants to weaken, divide and destroy Russia.

U.S. President Joe Biden criticized Putin’s overt nuclear threats against Europe. Meanwhile, NATO Secretary-General Jens Stoltenberg downplayed the threat, saying Putin “knows very well that a nuclear war should never be fought and cannot be won.” This is not the first time Putin has invoked nuclear weapons in an attempt to deter NATO.

I am an international security scholar who has worked on and researched nuclear restraint, nonproliferation and costly signaling theory applied to international relations for two decades. Russia’s large arsenal of tactical nuclear weapons, which are not governed by international treaties, and Putin’s doctrine of threatening their use have raised tensions, but tactical nuclear weapons are not simply another type of battlefield weapon.

Tactical nuclear weapons, sometimes called battlefield or nonstrategic nuclear weapons, were designed to be used on the battlefield – for example, to counter overwhelming conventional forces like large formations of infantry and armor. They are smaller than strategic nuclear weapons like the warheads carried on intercontinental ballistic missiles.

While experts disagree about precise definitions of tactical nuclear weapons, lower explosive yields, measured in kilotons, and shorter-range delivery vehicles are commonly identified characteristics. Tactical nuclear weapons vary in yields from fractions of 1 kiloton to about 50 kilotons, compared with strategic nuclear weapons, which have yields that range from about 100 kilotons to over a megaton, though much more powerful warheads were developed during the Cold War.

For reference, the atomic bomb dropped on Hiroshima was 15 kilotons, so some tactical nuclear weapons are capable of causing widespread destruction. The largest conventional bomb, the Mother of All Bombs or MOAB, that the U.S. has dropped has a 0.011-kiloton yield.

Delivery systems for tactical nuclear weapons also tend to have shorter ranges, typically under 310 miles (500 kilometers) compared with strategic nuclear weapons, which are typically designed to cross continents.

Because low-yield nuclear weapons’ explosive force is not much greater than that of increasingly powerful conventional weapons, the U.S. military has reduced its reliance on them. Most of its remaining stockpile, about 150 B61 gravity bombs, is deployed in Europe. The U.K. and France have completely eliminated their tactical stockpiles. Pakistan, China, India, Israel and North Korea all have several types of tactical nuclear weaponry.

Russia has retained more tactical nuclear weapons, estimated to be around 2,000, and relied more heavily on them in its nuclear strategy than the U.S. has, mostly due to Russia’s less advanced conventional weaponry and capabilities.

Russia’s tactical nuclear weapons can be deployed by ships, planes and ground forces. Most are deployed on air-to-surface missiles, short-range ballistic missiles, gravity bombs and depth charges delivered by medium-range and tactical bombers, or naval anti-ship and anti-submarine torpedoes. These missiles are mostly held in reserve in central depots in Russia.

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