Fulfilling one of Kyiv's biggest requests to Washington, the United States will send its most advanced and highly sought-after air defense weapon, the Patriot missile system, to Ukraine to block Russia's air assault.

President Joe Biden announced Wednesday that the United States would provide the system as part of a $1.85 billion military aid package with training and other munitions - a deployment that follows 300 days of war and an onslaught of Russian missile and drone strikes that have in recent months increasingly targeted Ukrainian cities and towns. One battery is far from a panacea, but the Patriot air defense system could dampen Moscow's apparent strategy to cripple Ukraine's power grid and civilian infrastructure.

Biden made the announcement during Ukrainian President Volodymyr Zelensky's highly orchestrated visit to Washington, his first trip abroad since the start of the war, where he emphasized the value of U.S. military support.

"This could take some time to complete the necessary training, but the Patriot battery will be another critical asset for Ukraine as it defends itself from Russian aggression," Biden said at a joint news conference with Zelensky.

The Ukrainian president agreed, calling it "the strongest element of this package," and later adding that Ukraine had asked for additional systems. "We would like to get more Patriots," Zelensky said. "We are in war, I am really sorry."

The Patriot battery - with a strike range of roughly 20 to 100 miles - will not protect the entire country, but experts say it does show a strong commitment by the United States.

"It's a very significant political statement as well as an improvement of Ukraine's defense capabilities," said Tom Karako, director of the Missile Defense Project at the Center for Strategic and International Studies.

Here's what to know about the air defense system:

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The Patriot is a sophisticated surface-to-air guided missile system that can detect and shoot down incoming missiles and aircraft, offering blanket protection to civilians and armies. Conceptualized in the 1960s to protect Europe from the Soviet Union and then deployed in the 1980s, the Patriot has become the Army's most sought-after air defense system, frequently operated around the world.

A Patriot battery is equipped with a truck-mounted launching system, with eight launchers that each hold between four and 16 ready-to-fire missiles, depending on the type of munition. The system, which 90 soldiers can operate, also uses radars, control stations and power generators.

It was most famously used during the Gulf War in 1991. The U.S. air defense system intercepted Iraqi Scud missiles aimed at Saudi cities.

"Troops on the ground watched as Patriot missiles arched into the dark skies and collided with incoming Scuds, which exploded in a rain of fiery debris," reported The Washington Post from Saudi Arabia in 1991.

Since then, the system has been deployed to Europe, the Middle East and the Pacific against the threat of attack from Iran, Somalia and North Korea.

Now, in Ukraine, the air defense system will be used to fend off a Russian bombardment that has left much of the country without power during a cold winter.

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The Patriot system will fortify and improve the capability of Ukraine's air defenses, filling in some gaps, Karako said. But the system has a limited range, will require training and must be supplemented by other defenses.

"The problem, of course, is the Russians are shooting a ton of missiles, and they are exhausting the supply of air defense interceptors," Karako said.

In the past month, Russia has amped up its drone strikes against Ukraine. Russian cruise missiles and Iranian-supplied drones have targeted critical infrastructure in Kyiv and other cities, to deprive Ukrainians of heat, electricity and water. At any given moment, 2 million to 10 million Ukrainians have been living in freezing temperatures without electricity.

At the same time, Iran has agreed to substantially increase the number of drones it is supplying to Russia, from hundreds to thousands, and Iran plans to provide ballistic missiles that can supplement Russia's dwindling stocks.

The United States has provided other tools to defend Ukrainian skies. Previous tranches of military aid included long-range rocket artillery systems and advanced electronic equipment that converts unguided aerial munitions into "smart bombs" that can strike front-line targets.

But the battery - which the Biden administration had resisted sending earlier - could be especially useful as the barrage of Russian attacks is exhausting the supply of air defense interceptors, leaving millions of Ukrainians without power. Russian officials have warned that the deployment of the air defense system could be seen as an escalation and "entail possible consequences."

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Ukrainian troops will need months of training to operate the Patriot system, a senior U.S. defense official told reporters Wednesday, though the Pentagon is eyeing ways to compress some aspects of the training. The U.S. military typically assigns 90 troops to operate one Patriot battery.

Experts have said training under normal conditions would take about half a year. "This is a complex system to operate and maintain," the defense official said.

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Sending the long-range air defense system to Ukraine will escalate the conflict, Maria Zakharova, the spokesperson for Russia's foreign ministry, said at a press briefing on Dec. 15. "We would like to say that the Western weapons supplied to Ukraine are legitimate targets for the Russian armed forces and that they will be either destroyed or seized," Zakharova said.

Karako disputed that the deployment of Patriot systems escalates the conflict. Instead, it may push the Kremlin to reconsider some aerial operations, he argued.

"The fact they don't like it is probably a testament to it being a capable system," Karako said.

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The United States has about 15 Patriot battalions, many of them deployed in Europe and the Middle East, and the U.S. soldiers who operate these systems are already among the most frequently deployed Army units. U.S. troops will also have to train Ukrainian forces on how to use and maintain the system at a base in Germany.

Given the scarcity, Karako said the United States has to be strategic when it considers where it may send its Patriot systems and its trainers, because they're in high demand across the globe.

"We have to deter attacks in a lot of places," Karako said. "One can surmise that if a Patriot battery goes to Ukraine it's probably not going to be going home anytime soon."

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The Washington Post's Alex Horton contributed to this report.