Risk and red lines in Ukraine

The horrors of the Russian invasion of Ukraine and the astounding heroism of the Ukrainian people have naturally created a groundswell of sympathy. Many call for escalation of Western assistance to Ukraine, either by creating a no-fly zone, initiating airstrikes or even using NATO ground forces. We are profoundly sympathetic to Ukraine but fear that these scenarios will unleash an unmanageable escalation leading to the use of nuclear weapons. It is important that President Joe Biden hold this line at the upcoming NATO summit.

Russia's stock of tactical nuclear weapons includes short-range missiles, nuclear artillery, nuclear landmines and more. Russian President Vladimir Putin's ordering of Russian nuclear forces to high alert should be taken seriously. The lowest level of use, tactical weapons, opens the gateway to escalation. James Mattis, defense secretary under President Donald Trump, testified to Congress that there is not "any such thing as a 'tactical nuclear weapon.' Any nuclear weapon used any time is a strategic game-changer."

Mattis' point is key: Tactical nuclear weapons introduce a qualitatively different level of uncertainty in understanding how each side will behave. The potential result of this uncertainty is preemption: One side moves to strategic nuclear war in anticipation of the other side doing the same.

We believe that the threshold for escalation to tactical nuclear weapons is reached in situations in which NATO forces kill Russians and Russians kill Americans; on the ground from the introduction of NATO soldiers or in the air by the enforcement of a no-fly zone. Since the 1950s, the U.S. and Soviet Union and then Russia have confronted one another in proxy wars across the globe. But in all this time, there has been no confrontation of troops. NATO forces on the ground or in the air would change that.

Any advocate for escalation to direct military conflict with Russia must be willing to accept the risks, including how the conflict will escalate. Throughout the Cold War, game theory was deployed in the analysis of the strategic situation. Game theoretic analysis, however, requires sufficient information to make probabilistic assessments of the likelihood of different outcomes.

While in the Cold War, both sides were usually able to successfully signal their intentions and predict how the other side would respond, the war in Ukraine is different. It is characterized by "ambiguity" — an inability for each side to make any kind of sharp likelihood assessment. Crossing the tactical nuclear threshold would change the already-unprecedented level of ambiguity present for each side in assessing the behavior of the other.

The Cold War did not lead to nuclear Armageddon largely because the two sides developed ways of communicating intentions and limits, thereby coordinating their behaviors and reducing ambiguity. This was possible because of formal exchanges such as treaties, strategic signaling and international norms of conduct. Coordination is fundamental to successful navigation of the threat of nuclear weapons. Putin's claim that sanctions are themselves an act of war has no resonance because no such common understanding of sanctions exists.

We are not suggesting a no-fly zone would directly produce an all-out nuclear exchange. However, it could unleash a sequence of escalation that ends so. Will Russia respond by attacking bases from which NATO planes are launched or try to interdict arms shipments on Polish soil? Will Putin conclude that the U.S. will escalate at each stage until regime change occurs in Russia? Will strategic nuclear weapons be used by either side? Once this norm in rules of engagement is broken, strategic ambiguities will multiply, and escalation that neither side wants may follow.

There are circumstances where NATO military intervention is appropriate: any Russian use of nuclear, chemical or biological weapons against civilian targets, for example. Such a step would itself violate norms of engagement and require an unprecedented response. Our judgment, however, is that actions such as a no-fly zone introduce a qualitative enhancement of the ambiguity associated with the U.S.-Russia conflict that can only further destabilize the situation, making what happens in response difficult and frightening to predict.

To avoid further destabilization, Biden should make explicit how the U.S. will respond if Russia uses nuclear or chemical weapons. This will reduce ambiguity about the consequences of use for both sides and their allies. If Russia crosses the nuclear or chemical threshold against civilians despite the clear signals the U.S. will have sent, only then would the establishment of a no-fly zone and introduction of Patriot defense systems, which require American operators, or the deployment of troops to set humanitarian zones, be justified.

Each of these actions would signal that civilian targeting is unacceptable, but leave in place another red line — no attacks on missile sites or air bases in Russia — to help minimize additional uncertainty that could lead to the kind of devastating escalation where there are no winners.