Ukraine has been able to effectively use a plethora of donated air-defense systems in an agile way to deny Russian air superiority during Moscow’s invasion, and continuing that defense will be key once Ukrainian forces begin a long-awaited new offensive in the fight, the U.S. Air Force’s top officer says.

Ukrainian forces are using high-end systems such as Patriot air defenses and National Advanced Surface-to-Air Missile Systems to defend Kyiv and other key locations effectively, and staying mobile to keep them operating. These defenses have shot down drones, cruise missiles and aircraft, U.S. Air Force Chief of Staff Gen. Charles Q. Brown, Jr., says.

“That actually puts a lot of fear into the Russian pilots, and they’re less inclined to venture in [to Ukrainian] locations,” Brown said June 7 at a Mitchell Institute for Aerospace Studies event. “And that’s where I see aspects of that air power playing into the counter offensive. It keeps Russian air power off the backs of the Ukrainians and it allows them to execute a bit better, being able to use their air defenses to their advantage.”

Ukraine has long said a counter offensive to push back Russian forces has been coming, and there have been conflicting reports that it already may have started on the war’s eastern front. The country’s leaders, especially President Volodymyr Zelenskyy and Defense Minister Oleksiy Reznikov, have been urging other nations to provide more weapons for the offensive.

During every meeting of the U.S.-led Ukraine Contract Group, a collection of dozens of nations that come together to determine how to provide additional aid for Kyiv, U.S. officials have said the most urgent need is ground-based air defenses.

“This contact group will continue driving hard to help Ukraine defend the skies. In recent weeks, Russia has intensified its sordid bombardment of Ukrainian cities and infrastructure,” U.S. Defense Secretary Lloyd Austin said at the last meeting, on May 25. “And the Kremlin’s cruelty only underscores Ukraine’s need for a stronger, layered ground-based air defense architecture.”

Brown says Ukraine’s success in using its air defenses to keep the skies contested demonstrates a need to think differently about base defense.

“That’s the thing that we as a joint team have got to think through,” he says. “In order to have air superiority, it’s not only how we attack an adversary in order to gain that air superiority, it’s also how we defend ourselves at our locations and bases.”