Co-authored by Anastassia Fedyk at UC Berkeley, Tetyana Balyuk at Emory University and Tania Babina at Columbia University

Since the beginning of a full-scale invasion into Ukraine on Feb. 24, 2022, Vladimir Putin’s military forces have been making increasingly indiscriminate attacks. Entire cities are left in ruins, some without electricity and running water, and millions of people have been sheltering for days on end in basements and metro stations. Hundreds of civilians are dead.

Amidst the bombing of residential areas in Kharkiv, Kyiv, Chernihiv, and Okhtyrka, among others, Ukrainian President Volodymir Zelenskiy has made progressively more urgent pleas for his allies to help “close the sky” over Ukraine. The latest “wake-up” plea came this morning, after the fire in the Zaporizhzhia nuclear power plant—the largest nuclear plant in Europe—gripped the world with fear of a nuclear disaster worse than Chernobyl.

So what does “closing the sky” over Ukraine actually mean, why would we consider doing it, and how would it work?

Closing the sky (also called “establishing a no-fly zone”) means saying “no” to certain types of aircraft over specified territories. Practically, the international community would ban foreign military aircraft from flying over the sovereign territory of Ukraine, or certain parts thereof.

Thirty-three countries—including the U.S., Canada, and all countries in the European Union—have already banned any Russian aircraft (including commercial flights) from their airspace in the past week. President Zelenskiy would like to do the same with the Russian military planes delivering air strikes to Ukrainian cities. Given Ukraine’s limited air capabilities, he cannot effectively guard Ukraine’s skies while warding off 150,000 Russian troops advancing around his cities on the ground. That is why he has asked his allies to help.

Why would Ukraine need this?

Ukraine’s pleas for a no-fly zone increased in proportion to the increasingly indiscriminate bombing of Ukrainian civilian infrastructure by the Russian forces. The reason is that it is difficult for Ukraine to effectively ward off the advancing Russian troops on the ground while Russian warplanes continue bombing civilians.

Closing off the air attacks would make it easier to focus the fighting on the military personnel while keeping the civilian population safer. This will help to preclude tragic losses of life. The Russian attack on the Zaporizhzhia power plant points to another reason for a no-fly zone over Ukraine: Europe cannot afford the nuclear fallout that would occur in the event of a more destructive explosion in any of Ukraine’s nuclear power plants.

How would a no-fly zone over Ukraine be implemented?

This would work just like trespassing laws. There would be a ban on military aircraft flying over Ukraine, and anybody violating that ban would face consequences. Practically speaking, the ban could go into effect sometime after the announcement—for example, within 12 or 24 hours—to eliminate any potential for ignorance or accident to serve as excuses. Of course, the ban will only be effective if the consequences for its violation are credible. This is why besieged Ukraine is not able to implement the ban by itself and seeks the support from the international community.

However, it’s important to realize that the idea is for those consequences to never need to pass. In game theory this corresponds to a key concept of “equilibrium”: when incentives are correctly set, even with all sides acting in their own interests, there is no crime and hence no need to punish. If there are real consequences for trespassing, then almost nobody trespasses. And if there are real consequences for violating Ukraine’s airspace, Russia will not do so.

Would “closing the sky over Ukraine” amount to entering a direct war with Russia?

No. Closing the sky over Ukraine is not an act of aggression against Russia, as it would apply only to the sovereign territory of Ukraine at Ukraine’s own request. Closing the sky to prevent an aggressor’s aircraft from bombing civilian infrastructure is not an act of war. In fact, it is an action specifically stipulated by the United Nations for the purposes of peace.

The UN Security Council has established no-fly zones on previous occasions. There is a growing need to do so now. Unfortunately, it appears increasingly evident that the impressive array of sanctions imposed on Russia in the past week has, thus far, proved inadequate to de-escalate Russia’s invasion of Ukraine. Instead, Putin’s military has shifted its focus to striking not only Ukrainian military targets but also administrative centers, apartment buildings, and civilian infrastructure.

To close the skies over Ukraine, not a single shot needs to be fired. Instead, the idea is to provide a credible threat of deterrence. If Russian military planes know that they will be shot down for violating the no-fly zone, then they will not fly over Ukraine’s land. Putin will likely test the allies’ resolve—Russian military jets have already violated Sweden’s airspace—and it will be important to show that the threat of deterrence is real and strong. If Putin somehow goes off the “equilibrium” path, he will be choosing to engage militarily in direct conflict with the international community well beyond Ukraine. In such a case, we would be better off knowing it now, and not when Putin enters Finland, Poland, or Latvia.