Both NATO and Russia would like to see the other reduce its stockpile of tactical nuclear weapons, but the two sides have been unable to agree on mutual reductions. Even modest progress on the issue at NATO’s Chicago summit seems unlikely. This is partially because it is unclear what a “tactical” nuclear weapon is.

Arms controllers have struggled for decades to create an effective definition for tactical (or nonstrategic) nuclear weapons, as opposed to strategic ones. It is, in fact, excessively difficult to distinguish between the two categories. The characteristics of warheads and delivery vehicles in the US arsenal overlap enough to preclude a capabilities-based definition, and this is not to mention Russia’s nuclear munitions.

If the United States and Russia are to pursue a new arms control treaty covering tactical weapons, arms control negotiators will need an elegant solution to the dilemma of defining a tactical nuke. Creating such a definition is about to get a lot harder. The United States plans to equip its tactical bombs with strategic employment options. Given this state of affairs, it may be time to simply drop the term “tactical nuclear weapon” and seek an arms control agreement based on the total number of warheads, regardless of their prior designation.

What Makes a Nuclear Weapon Tactical? For practical purposes, tactical nuclear weapons are thought of as nuclear weapons systems with shorter ranges and smaller yields than those with intercontinental missions. Of these, it is estimated that the US has 400 warheads and Russia has approximately 2,000. But most unambiguously tactical battlefield weapons — like nuclear artillery shells or demolition munitions — have long been retired. For the few remaining weapons in the US tactical arsenal, the characteristics of tactical and strategic nuclear weapons overlap enough to render the term tactical all but useless as an arms control definition.

The confusion covers nearly every element of definitions for tactical nuclear weapons. One might think of a weapon as strategic if it has the range to hit the territory of the United States or Russia when launched by the other. But all operational US tactical nuclear weapons violate the range distinction. The tactical US B-61 nuclear bombs deployed on the territory of NATO allies in Europe could, if the fighter-bombers delivering them were refueled in air, reach targets in Russia. These fighter-bombers do not count against New START. The soon-to-be-retired nuclear variant of the Tomahawk land attack cruise missile also had the potential to hit targets in Russia from naval platforms.

Tactical nuclear weapons are sometimes assumed to have less explosive power or yield than strategic warheads. For example, the US B61-3 gravity bomb deployed in Europe has a variable yield option that can produce an explosion as small as 0.3 kilotons, or one-fiftieth the yield of the bomb used on Hiroshima. But dialed to its maximum yield, this same B61-3 can also produce a 170-kiloton blast, which is greater than the 100-kiloton yield of the submarine-launched, and strategic, W76 warhead.

US-Russia arms control treaties have controlled classes of weapons by limiting types of delivery vehicles, thereby reducing the numbers of warheads associated with them. The Strategic Arms Reduction and Intermediate-Range Nuclear Forces treaties gave precise definitions of what delivery vehicles they limited or banned. This approach will not work easily for tactical nuclear weapons. The United States and Russia deploy these nuclear warheads with dual-use systems — chiefly fighter-bombers — that can deliver conventional or nuclear warheads. To limit these sometimes-nuclear delivery vehicles, the US and Russia would have to accept controls on what are predominantly conventional systems or accept highly intrusive inspections to confirm that systems are not deployed with nuclear warheads. Both solutions are technically feasible, but neither is an easy sell in Moscow, Washington, or Brussels.

Some definitions for tactical nuclear weapons consider the intended target of the weapon. As the Defense Department thinks of them, tactical weapons are intended for use with immediate effect against an enemy’s war-fighting capacity in support of a military operation of limited scope. A strategic weapon is intended for use against an enemy’s broader war-making capacity (i.e., nuclear installations and economic and political targets). These definitions may work for military planning, which generally thinks of weapons in terms of their effects. But targeting intentions are too fluid for use in arms control definitions.

Avoiding the endless hazards of finding a comprehensive definition of a tactical nuclear weapon, US and Russian policy experts have operated with a definition based on what tactical nuclear weapons are not. They exist outside of the INF Treaty and are not covered by New START, which limits strategic nuclear weapons. It follows, under this “exclusion definition,” that all other weapons in the stockpile must be nonstrategic, or tactical, nuclear weapons. A variety of weapons systems fall under this exclusion, including warheads for bombers, short-range missiles, cruise missiles, and anti-ballistic missile and anti-aircraft systems. This is the most functional solution currently available, but it does not provide the exactitude that most arms control treaties enjoy.

Tactical Goes Strategic. It is about to get harder to differentiate between a tactical and a strategic nuclear warhead. Under the proposed B-61 life extension program, the last bombs in the US tactical nuclear arsenal will end up with smaller maximum yields and options for deployment with strategic bombers. The result will be one warhead design, the B61-12, that could be deployed on B-52 and B-2 bombers and on F-35, F-15, and F-16 fighter-bombers. As Hans Kristensen of the Federation of American Scientists wrote, the B61-12 could end the US practice of designating nonstrategic warheads: “The United States has eliminated all but two of its nonstrategic nuclear weapons, has decided to retire one of them [the Tomahawk], and appears to be on a path toward phasing out designated nonstrategic nuclear warheads from its stockpile altogether.”

This new bomb could make it easier to define an arms control category of tactical nuclear warheads based on yield. By removing the B61-3 from the US nuclear arsenal, the B61 life extension program will eliminate the overlap between the yields of US tactical and strategic warheads. At 50 kilotons, the new B61-12 will have the smallest maximum yield in the US arsenal. This would allow the United States to comply with an arms control definition to set a yield threshold — say 50 kilotons or below — under which all warheads count as nonstrategic. It is unclear if such a definition could work for Russia.

Functionally, however, folding the B61-12 into one of two categories of nuclear weapons seems absurd. The B61-12 could concurrently have tactical and strategic missions while deployed at air bases from Missouri to Turkey.

Counting the Bomb. It may not be necessary to create a firm definition for tactical nuclear weapons in a future arms control treaty. Many analysts, including Steven Pifer of the Brookings Institution, argue that the United States and Russia could negotiate a single limit on all nuclear warheads, perhaps with a sublimit covering deployed strategic warheads, allowing the countries to set their own ratios of strategic to tactical nuclear warheads.

In determining the right mix of warheads, the United States would likely opt to reduce its stockpile of non-deployed strategic warheads and Russia would opt to reduce its stockpile of tactical warheads. Neither the US non-deployed upload capacity nor Russia’s stockpile of tactical weapons would go away entirely.

This elegant work-around has the benefit of not requiring difficult and faulty definitions of tactical nuclear weapons, while facilitating steep cuts in tactical warheads. On the downside, by seeking indirect limits on tactical nuclear weapons, it would not address the disparity between US and Russia tactical stockpiles to a degree that some US lawmakers might prefer. Russia would likely retain a tactical stockpile several times greater in size than the remnants of the US tactical arsenal. This proposal, however, could set a valuable precedent of limiting total arsenals — a precedent that will be important for any future negotiations involving other nuclear powers.

In any case, as a bonus for ratifying a single-limit treaty, the United States and Russia would be one step closer to retiring the term “tactical nuclear weapon,” allowing this confused Cold War anachronism to drift into irrelevance. At long last, a nuclear weapon by any other name would be limited, along with all the rest.