The first warning was a blip, a small anomaly picked up by radar scanning the skies over Ukraine. Within seconds, it became clear that the blip was a Russian ballistic missile streaking in Kyiv’s direction at several times the speed of sound.

It was just before 4 a.m. on Dec. 11, and there was no time to sound air-raid alarms in the city. While millions of civilians slept, Ukrainian forces fired off several American-supplied Patriot missiles as the deadly battle in the sky commenced.

Missile-on-missile battles like this play out in a matter of minutes, said a Ukrainian major, Volodymyr, the commander of a Patriot air-defense battery who insisted that only his first name be used because of the sensitivity of his unit’s operations.

From a mobile control room near Kyiv, his team tracked the salvo of incoming Russian missiles as the Patriot’s algorithms calculated their speed, altitude and intended course. With shuddering booms and bursts of light, its interceptor missiles knocked down one Russian missile after another.

“Given that the Patriot is one of the few systems that can effectively shoot down ballistic missiles, and ballistic missiles cause the most casualties, I think the number of lives saved during the war is in the thousands,” Major Volodymyr said.

That night was a success, but more recent missile barrages have done more damage as Russia steps up its assaults, searching for new combinations of weapons and trajectories to evade Ukrainian defenses. Those attacks have underscored even more acutely Ukraine’s urgent need for air defense.

On Dec. 29, Russia fired more than 120 missiles at cities across Ukraine, killing at least 44 people, including 30 in Kyiv, the capital. On New Year’s Eve, Ukraine’s forces said they had shot down 87 of 90 drones aimed at targets around the country. And on Tuesday, according to the Ukrainian military, Russia fired at least 99 missiles and 35 drones at Kyiv and other cities, killing at least five people and injuring dozens.

In aerial assaults in just that five-day span, United Nations observers documented 90 civilian deaths, including two children, and 421 civilian injuries. And President Volodymyr Zelensky of Ukraine said on Tuesday that Russia had fired more than 500 missiles and drones at targets across the country in that time.

“There is no reason to believe that the enemy will stop here,” Gen. Valery Zaluzhny, Ukraine’s top commander, said on social media after Tuesday’s attack. “Therefore, we need more systems and munitions for them.”

But White House and Pentagon officials have warned that the United States will soon be unable to keep Ukraine’s Patriot batteries supplied with interceptor missiles, which can cost $2 million to $4 million apiece.

Since the start of the war in February 2022, Russia has directed more than 3,800 drones and 7,400 missiles at Ukrainian towns and cities. At the same time, Ukraine has become a testing ground for an array of air-defense systems, according to the Ukrainian military.

They range in sophistication from truck-mounted Stingers and short-range antiaircraft guns, like the German-made Gepards, to complex systems with longer ranges, like the French-designed SAMP/T, which can hit a target 60 miles away. There is also the National Advanced Surface-to-Air Missile System, or NASAMS, which is jointly produced by the United States and Norway.

Only the Patriots are designed to counter ballistic missiles, and from the moment the first Patriot battery entered the combat space, they reshaped the battle for the skies.

Major Volodymyr, 32, was manning a Soviet-era S-300 system when Russia launched its invasion in 2022. Yet while Ukrainian air-defense teams managed to keep Russian fighter jets from gaining dominance in the air and put up an agile defense against cruise missiles, they had nothing designed to shoot down ballistic missiles.

As Russian strikes ravaged critical infrastructure across Ukraine, officials contemplated evacuating Kyiv that November, and the United States Congress approved the first Patriot battery for Ukraine a month later.

Major Volodymyr was part of a team dispatched to Fort Sill, a former frontier cavalry post in southwestern Oklahoma, for a 10-week course on how to operate and maintain the system.

“We quickly found a common language with the Americans,” he said in a recent interview. “We are constantly in touch with them. If something happens, they worry, write, congratulate us.”

After two further weeks of training in Poland, he traveled to Ukraine with the first Patriot system. Within days, his team was put to the test in combat.

On May 4, Russian forces fired a hypersonic missile at Kyiv. And although President Vladimir V. Putin had deemed the weapon “unbeatable,” a Patriot interceptor missile shot it down.

“It was quite unexpected,” Major Volodymyr said. “We had just arrived from training and did not fully understand what exactly we had destroyed.”

“Later, when we found out, our confidence in the equipment that our partners provided us grew,” he said.

In May and June, during some of the most complex attacks involving drones, cruise missiles, ballistic missiles and hypersonic missiles, Ukraine’s two Patriot batteries shot down all 34 ballistic missiles that Russia had fired at Kyiv, according to a report by the Center for Strategic and International Studies, a Washington-based research organization.

“There were days when the guys barely had time to reload the launchers,” Major Volodymyr said.

Just as important is the role the Patriots have played in defending against sophisticated saturation bombardments. Those assaults use a combination of land, sea, and air-launch platforms to send missiles and drones streaming into Ukraine along varied flight paths, descending along different trajectories with coordinated impact times intended to overwhelm Ukraine’s defenses.

In just one such recent bombardment, Russia sent missiles flying past Kyiv only to have them circle back to attack.

Russian forces also use decoys and program missiles to change course during their flight to confuse air-defense crews.

But the Patriot’s powerful radar has a range of over 93 miles and can track up to 100 targets at once, according to a report by the Congressional Research Service. Its radar also provides missile guidance data for multiple interceptor missiles, according to the report, and is resistant to electronic jamming.

Over the past year, Ukraine has created “a unified system of interaction” that allows air-defense teams using different systems to use information collected by the Patriot crews and other sophisticated radar arrays, said Lt. Col. Liubov Kynal, a spokeswoman for Ukraine’s central air-command wing.

“We all work as one organism,” she said.

The truck-mounted command center — which calculates trajectories for the interceptors, controls the launching sequence and allows soldiers to communicate with other air-defense units — is the only manned part of the system.

“Of course, we are constantly moving the system, constantly changing locations so that the enemy does not know where we are,” Major Volodymyr said.

The battery’s other major parts, including power stations, missile launches and radar arrays, are mobile and move frequently to avoid detection.

“We have a shift constantly on the equipment and ready for immediate work,” the major said.

While a Patriot battery requires a minimum of 70 trained soldiers to run and maintain, only two or three soldiers are needed in the control station to operate it in combat.

“When the alarm goes off, the full combat team arrives,” Major Volodymyr said. They can assemble in under five minutes, he said.

Still, the protection provided by the Patriots is limited, like a blanket that covers only a fraction of a bed. “We were able to defend Kyiv, but at the same time Odesa was being destroyed,” Major Volodymyr said.

Ukrainian commanders are now trying to plan for a future without knowing what weapons they may have at their disposal.

“We managed to create a shield over the state thanks to our foreign partners,” Major Volodymyr said. “But if our foreign partners turn their backs on us, we will return to the beginning of the war, when people simply did not come out of their shelters and the Russians tried to turn our cities into complete ruins.”