There is no "clean solution"

Many laypeople imagine a no-fly zone as a "clean" solution: Let the skies over Ukraine be patrolled, therefore denying the Russians the opportunity to exploit their own air superiority. But that is a romanticized notion of what a no-fly zone actually is.

U.S. Air Force officers Mike Pietrucha and Mike Benitz explained in a recent piece on their website War on the Rocks that "a no-fly zone is not a military half-measure."

They note that such a "zone" is in reality "a combat operation designed to deprive the enemy of its airpower, and it involves direct and sustained fighting." And those would mean direct confrontation between NATO and Russian forces, and thus the risk of extending the war throughout Europe.

Not only would NATO planes have to attack Russian fighters in case of doubt in order to maintain the no-fly zone, they would also have to take out Russian air defense positions on the ground. Some of Russia's modern S-300 and S-400 air defense systems would not even be on Ukrainian soil, but would have to be attacked in Russia or Belarus , from where they cover large parts of Ukrainian airspace.

Weapons deliveries could backfire

Some say that the West is already involved in the war with its continuous deliveries of weapons, and that additional NATO military actions in support of Ukraine would change little.

But that is actually not the case under international law. The Charter of the United Nations does allow the international community to help states under attack that are merely exercising their right to self-defense.

Arms deliveries in support of Ukrainian self-defense are, however, an entirely different category compared to when a state or a community of states participates in hostilities.

In that case, a third state becomes a party to the war — as Belarus, for example, has become simply by making its territory available to Russia for the attack on Ukraine. At present, however, the West fears nothing more than being drawn into this conflict and facing Russia directly in a war.

Respecting international law is strategic too

Russia has not only violated international law by starting a war. It is also violating international law with its targeted attacks on civilians. Russia's President Vladimir Putin has concocted his justifications for the war: that Ukraine is somehow ruled by Nazis and is committing genocide against Russian-speaking Ukrainians, which is simply a lie.

Why does it matter, then, whether Putin is given further reasons for war against NATO as well? If he needed to, he would make those up too. On Saturday, for example, Putin described Western economic sanctions against Russia as a declaration of war by the West. So why should we insist on the niceties of international law?

Putin might feel constrained to enter an escalation spiral.

That is too simplistic a reading of international law. If the West interferes more directly, Putin might have to take it as a declaration of war — even if he hadn't at all been intent on a head-to-head conflict with the West. Then he might feel constrained to enter an escalation spiral, which could lead to a bonafide third world war in Europe, which could end in a nuclear escalation .

Second, it would jeopardize the hard-won unity of the West and NATO in the Ukraine crisis. It is hardly conceivable that all NATO states would agree to military intervention in Ukraine, whether in the form of a no-fly zone or otherwise.

Fighter jets as support

On Saturday, Ukrainian President Volodymyr Zelensky made an alternative proposal to the no-fly zone in a Zoom call with U.S. members of Congress: Ukraine could instead be supplied with aircraft, he demanded.

"His main ask was for the U.S. to allow Poland and Romania to transfer Soviet era jets to Ukraine, and for the U.S. to compensate by giving more advanced planes to those two NATO allies," Democrat Congressman Brad Sherman wrote on Twitter after the meeting.

It was an idea that EU foreign affairs envoy Josep Borrell had introduced a week ago, but it has not yet materialized — to the astonishment of some experts. Indeed, Soviet jets from old stocks of Eastern European states would allow Ukrainian pilots to use them straight away, as they were trained on the same aircraft.

"I fully understand and share the reasons why NATO does not want to reach a no-fly zone," says security expert François Heisbourg of the Foundation for Strategic Studies (FRS) in Paris. "But I don't understand why the U.S. and perhaps others are blocking the transfer of fighter aircraft to Ukraine. This is not seen as an act of war in international law and practice."

Wider geopolitical risks

Indeed, Zelensky's appeal seems to have created renewed interest around this issue.

On Sunday, U.S. Secretary of State Antony Blinken said they are "looking actively now at the question of airplanes that Poland may provide to Ukraine and looking at how we might be able to backfill should Poland decide to supply those planes."

If Poland gives up its own fighter jets, it could become Russia's next target.

Poland's Prime Minister Mateusz Morawiecki initially rejected the idea, saying: "Poland will not send its fighter jets to Ukraine, nor will it allow them to use its airports," he wrote on Twitter.

The reaction caused surprise — after all, Poland itself had made a push for the delivery of fighter jets as recently as last Thursday. Within a few hours, NATO Secretary General Jens Stoltenberg flew to Poland, and the Polish government backed down.

It is also possible that a dispute over compensation for the Polish deliveries is at play. Poland understandably does not want to weaken its own air force in a situation of tension. If it now gives up its own fighter jets, the country could become the next target of Russia's expansionist ambitions.

Warsaw, which is already in the process of replacing its old Soviet-built jets with modern Western ones, may want to persuade the U.S. to quickly hand over such modern jets to Poland in exchange for those that would be left for Kyiv.