While Ukraine has put up a valiant fight against Russia’s invasion thus far, there is a lingering fear that it’s only a matter of time before Russia’s superior military overwhelms it. And increasingly in recent days, this has led to calls for what might sound like a middle ground for the Western countries that have declined to send ground troops: a no-fly zone over Ukraine, which could significantly damage Russia’s ability to attack.

The reality is far less simple and less likely to be a true middle ground.

While the idea has been percolating in some corners for the better part of a week, Ukrainian President Volodymyr Zelensky signed on to it Monday. He told Axios that Ukraine “can beat the aggressor” if Western allies “do their part.” Other Ukrainian leaders are calling for it, as well as some conservative politicians in Britain. A Ukrainian journalist pleaded with British Prime Minister Boris Johnson for a no-fly zone Tuesday in an emotional scene. Rep. Adam Kinzinger (R-Ill.) called for one Friday. And a former NATO supreme commander, retired U.S. Air Force Gen. Philip Breedlove, has said its time has come.

Thus far, the calls have fallen on deaf ears. NATO, of which Ukraine is not a member, has reinforced that it will not become so directly involved. Governments including in Britain, the United States and Poland, which neighbors Ukraine, have dismissed the idea in varying terms. Johnson responded to the impassioned plea from the Ukrainian journalist Tuesday by saying he was “acutely conscious that there is not enough that we can do, as the U.K. government, to help in the way that you want.”

White House press secretary Jen Psaki seemed on Monday, though, to stop somewhat short of completely ruling it out. She instead detailed the reasons it is best to be avoided:

PETER ALEXANDER: On military, is there any way in which the U.S. would support a no-fly zone over Ukraine?

PSAKI: Well, here’s what important for everybody to know about a no-fly zone: What that would require is implementation by the U.S. military. It would essentially mean the U.S. military would be shooting down planes – Russian planes. That is definitely escalatory. That would potentially put us into a place where we’re in a military conflict with Russia. That is not something the president wants to do.

KRISTEN WELKER: So, that’s a no on that.

PSAKI: Those are all the reasons why that’s not a good idea.

At the daily White House news briefing the same day, Psaki again declined to take the option off the table completely, but she did indicate that it would contradict Biden’s pledge not to risk American lives. “I think what’s important to note here,” she said, “is essentially what this would be a step toward, because a no-fly zone would require implementation.”

This is the key point about no-fly zones, experts say: They are quite likely to lead to direct military engagement, depending on the circumstances, and must be entered into accordingly.

The United States and allies have deployed them in recent decades, particularly over Iraq after the Persian Gulf War, Bosnia in the 1990s and Libya in 2011. As a 2013 Congressional Research Service report noted, these efforts met with varying degrees of success.

“Risks are both intentional escalation and inadvertent incident that could ratchet up,” Bruce W. Jentleson, a political scientist and public policy expert at Duke University, said in an email. “The main reasons U.S. and allies didn’t intervene in Hungary 1956 or Czechoslovakia 1968 were risks of direct military clashes between us and Soviets/now Russians. All told, there are unprecedented risks of escalation, especially nuclear, that even if [Russian President Vladimir] Putin hadn’t made his threats would have been there.”

There are practical considerations beyond that, said U.S. Naval Academy professor Stephen Wrage, the co-author of a 2017 book on no-fly zones. For one, countries would need bases from which to operate thousands of flights, potentially from Germany or Poland. He said that, beyond Russia’s military might, surface-to-air missiles are better today, so “Putin’s planes would not be forced out of the sky at the outset the way” planes were in Iraq, Bosnia and Libya. There’s also how countries handle myriad potential triggers for a head-to-head confrontation.

Finally, Wrage argued that involving the West so directly could damage the effort to win hearts and minds.

“The one thing an American/NATO no-fly zone surely would accomplish: It would demolish the dramatic display of Ukrainian valiant resistance to gross and undeniable Russian aggression,” Wrage said, concluding: “There are many other ways to express solidarity with the people of Ukraine.”

At least one proponent, Breedlove, acknowledges much of Wrage’s observations. In an interview with Foreign Policy, he said that many calls for no-fly zones are overly simplistic and agreed that establishing a no-fly zone would be akin to carrying out an act of war. But he argued that the moment still called for it:

BREEDLOVE: I am actually a proponent of it. But let me now tell you why it will probably not happen: because the reality of a no-fly zone is — it is an act of war. There are a lot of people who don’t understand no-fly zones. You don’t just say, “That’s a no fly zone.” You have to enforce a no-fly zone, which means you have to be willing to use force against those who break the no-fly zone. The second thing, which nobody understands, is if you put a no-fly zone in the eastern part of Ukraine, for instance, and we’re going to fly coalition or NATO aircraft into that no-fly zone, then we have to take out all the weapons that can fire into our no-fly zone and cause harm to our aircraft. So that means bombing enemy radars and missile systems on the other side of the border. And you know what that means, right? That is tantamount to war.

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Q: Yet, in spite of all of that, you said you would actually support the idea of a no-fly zone?

BREEDLOVE: Are we going to sit and watch while a world power invades and destroys and subjugates a sovereign nation? Are we just going to watch? I mean, a friend recently said, “This is like biblical times, and the whole Colosseum is watching the lions and the Christians, and they’re pulling for the Christians, but they just watch.” So the question is, is the West going to tolerate Russia doing this to Ukraine? What if the Russians do what they did in eastern Syria and they drop barrel bombs and make rubble of cities and terrorize citizens and force them on the road and make them refugees across Europe? Where is the line that Russia crosses in its inhumanity such that the rest of the world reacts?