What is a ‘no-fly zone’ and how could it escalate the Ukraine-Russia conflict?

Zelenskyy says a no-fly zone—which would designate certain areas of Ukraine as off-limits to Russian military aircraft and deploy force to keep them out—would help Ukraine defeat Russia “with much less blood” than if Ukrainian forces were left to fight without support from the U.S. or NATO.

But imposing a no-fly zone over a country doesn’t automatically end violence. In fact, both the U.S. and NATO are concerned that declaring a no-fly zone over Ukraine could actually make the situation worse, once their own military forces are called on to enforce it.

On Monday, U.S. Sen. Marco Rubio (R-Fla.) warned enforcing a no-fly zone over Ukraine “would mean [the start of] World War III,” since the move could require U.S. forces to directly engage with Russian aircraft. That escalation could possibly provoke Russia President Vladimir Putin to retaliate with more drastic—and, maybe, nuclear—action.

What is a no-fly zone?

In the years since the Cold War ended, the U.S. and NATO have often used no-fly zones as a relatively cost-free way to deny airspace to militaries that were, frankly, much less powerful than their own.

During the Bosnian War in 1992, the United Nations Security Council (UNSC) prohibited unapproved military aircraft from flying into Bosnian airspace to clear the way for delivery of humanitarian aid. During the First Libyan Civil War in 2011, the UNSC imposed a no-fly zone over Libya to protect civilian populations from airstrikes and missiles lobbed between the warring factions.

In both situations, NATO aircraft enforced the no-fly zone. They were responsible for shooting down or warding off all transgressors. In both cases, NATO’s air warfare capability far outstripped that of the aggressors trying to break through. The potential repercussions for the powers enforcing the no-fly zones were also fairly low. Neither Bosnia or Libya had any ability to attack far beyond their own borders and retaliate against the U.S. or NATO allies.

But a no-fly zone over Ukraine would be different.

Russia has the world’s second-largest air force after the U.S., meaning gaining aerial superiority would be much harder for the U.S. or NATO to accomplish. Without dominance in the skies, a military won’t be able to enforce a no-fly zone.

“Not a magical umbrella”

Enforcing a no-fly zone over Ukraine would mean denying Russian military aircraft access to a given airspace and stopping any Russian attempt to bomb opposing forces, engage in reconnaissance, or fight for aerial superiority.

“It would essentially mean the U.S. military would be shooting down Russian planes,” White House spokesperson Jen Psaki told MSNBC on Monday. An escalation of that sort would violate promises U.S. President Joe Biden made Thursday, when he said that American forces “are not and will not be engaged in the [Russia-Ukraine] conflict.”

Militaries enforcing the no-fly zone would also need to “take out all the weapons that can fire into our no-fly zone and cause harm to our aircraft,” says former NATO Supreme Commander Philip Breedlove to Foreign Policy. That means a no-fly zone would lead to direct military contact between Russian and NATO forces and mark a significant escalation in the conflict.

Breedlove—who supports imposing a no-fly zone, despite its risks—says “many in the world would interpret [a no-fly zone] as an act of war.”

If U.S. troops do clash with the Russian military, there’s an unsettling risk that the war would spin into a nuclear conflict—especially since Russian President Vladimir Putin placed Russia’s nuclear forces on “high alert” Sunday. Putin said he was readying his nuclear warheads for launch in response to nonspecific “aggressive statements” from NATO.