Why Experts Are Growing Alarmed About Ukraine's Air Defenses

“If Ukrainian air defense starts getting eroded to the point where they don’t have the capacity to engage, you could see Russian aircraft coming back into Ukrainian airspace,” says Ian Williams, deputy director of CSIS’s Missile Defense Project. “That’s dangerous because… Ukraine’s success in the war so far has been enabled by Ukraine’s air defenses.”

If Russian planes assume dominance in Ukrainian airspace, a lot is at stake. Not only will it be harder for Ukraine to launch offensive and defensive ground campaigns, it will also make it harder for Ukraine to move men and supply lines across the country, protect critical infrastructure, and withstand deadly bombing campaigns.

The leaked documents, which surfaced on social media sites last week, contained a trove of U.S. national security details related to Ukraine, China, and the Middle East. Some military analysts have cautioned against the veracity of the documents.

Col. Yuri Ihnat, a spokesperson for the Ukrainian Air Force, has said that Ukraine is facing serious challenges in protecting its airspace and needs assistance from its Western allies. “If we lose the battle for the skies, the consequences for Ukraine will be very serious,” he said, according to the Wall Street Journal.

Ukraine began the war with the largest ground-based air defense arsenal in Europe, excluding Russia, CSIS’s Williams says. The Soviet-made Buk and S-300 missile systems used to intercept Russian projectiles have made up most of Ukraine’s air defenses. But missiles for the Buk and S-300 systems are expected to be depleted by mid-April and May 3, respectively, according to one of the leaked Pentagon documents. The estimate is based on consumption rates at the time the document was published in late February.

“They had hundreds of launchers and thousands of interceptors,” Williams says. “It was old Soviet stuff they inherited but it worked and Russians in the initial invasion failed to destroy most of that.”

Russian aircraft breached deeper into Ukrainian territory at the start of the war to drop bombs but were quickly shot down and took heavy losses. Within the war’s first few months, Russia’s air force decided it couldn’t sustain such heavy losses, opting largely for long-range cruise missiles that the Buk and S-300 were focused on intercepting, experts tell TIME.

They warn that missile supplies for the S-300s and Buk are a profound challenge for Ukraine. “If stockpiles do run critically low, that gives [Russian planes] much more freedom to operate over the front lines,” says Sidharth Kaushal, a research fellow at the military sciences team at the Royal United Services Institute.

Russia has found it difficult to get powerful explosives onto targets in a sustainable, cost-effective manner. Cruise missiles are expensive and hard to produce. Russia has a lot of bombs that need to be on aircraft and flown above the target. “If they are suddenly able to do that at an acceptable aircraft loss rate, that changes a lot and opens up huge reserves of Russian bombs,” Williams says. “They would be able to do more damage, more quickly.”

Ukraine may soon start making tough choices about what Russian threats to engage. “That’s not a good situation to be in,” Williams says.

“The Ukrainian air defense network is still one that’s predominantly comprised of Russian/Soviet made capabilities but with some Western additions as the war has gone on,” Kaushal says.

Over the course of the war, Ukraine has also received Western-made defense systems. That includes National Advanced Surface-to-Air Missile System(NASAMS) from the U.S. and IRIS-T air defense batteries from Germany. This month, Ukrainian soldiers are finishing training on the Patriot missile system, which is considered the most advanced ground-based air defense system the U.S. has. Biden authorized the deployment of one Patriot missile battery last December. Ukraine is also planning to receive a Patriot battery from Germany.

The longer term goal is that the Western supplies could eventually replace the Soviet systems as they ran out, Williams says. “These are systems we can replenish; we make them,” he adds.

While Western-made systems can help Ukraine maintain strong air defense, these systems are “relatively limited in number” and there’s a “considerable training burden,” Kaushal says.

One alternative is for Western allies to reach out to countries that operate variants of the Russian systems to try and procure interceptors for resale. (Western interceptor missiles are not compatible with the S-300 and Buk.)

This is something Ukraine’s allies have already tried. “Allies have been trying to replenish those systems as well as scouring the planet trying to find any compatible interceptors that we could give them,” Williams says. “A lot of Eastern European countries have basically emptied their arsenals of those things.”

Another way to help Ukraine protect its airspace would be to provide F-16 fighter jets, which Biden has ruled out. “That will help a lot,” Williams says.

Poland and Slovakia announced in March that they would provide Ukraine with MiG-29 fighter jets.

“(F-16s) can be effective cruise missile killers,” Williams says. But operating F-16s can bring its own challenges as it requires special maintenance and training.

In the meantime, the Biden administration has been more willing to help replenish stocks of interceptors ahead of an expected spring Ukrainian offensive; it said last week that it would send additional interceptors and munitions as part of a $2.6 billion aid package.