They are in high demand, short supply and considered one of the most advanced US air-defence systems.
But despite allies around the world vying to get their hands on the long-range Patriot missile-defence systems, the Biden administration has finally agreed to send one to Ukraine.
"Over the past three hundred days, the Kremlin has tried and failed to wipe Ukraine off the map," said US Secretary of State Antony Blinken in a statement announcing the US$1.85 billion ($2.75 billion) aid package.
"Today’s assistance for the first time includes the Patriot Air Defense System, capable of bringing down cruise missiles, short range ballistic missiles, and aircraft at a significantly higher ceiling than previously provided air defence systems."
The announcement came after months of urgent requests from Kyiv for more robust weapons to shoot down Russian missiles.
With only a limited number of the systems, the US had been reluctant, only sending Patriots to protect neighbouring NATO countries like Poland.
The Patriot is intended to help Ukraine defend against the relentless missile and drone attacks that have pounded the country's energy infrastructure and plunged cities into darkness.
It is the most advanced surface-to-air missile system the West has provided Kyiv, but military analysts say the system itself is not the "game-changer".
"It's not really about the air defence, it's not really about the missiles — this is much more of a political thing," John Erath, senior policy director for the US Center for Arms Control and Non-Proliferation, told the ABC.
The decision is a potentially pivotal move in the conflict, which shows the United States' deepening commitment to Ukraine.
It may also indicate a broader shift in the West's willingness to send more sophisticated weapons systems to the battlefield.
Patriots 'a significant step'
The Patriot uses radar systems that have around three times the range of other air defences provided by the West, such as NASAMS and the IRIS-T.
Each Patriot battery consists of a truck-mounted launching system with eight launchers that can hold up to four missile interceptors each, a ground radar, a control station and a generator.
They can protect an area of up to 100 kilometres and can target aircraft, cruise missiles and ballistic missiles.
Stephan Fruehling from the ANU Strategic and Defence Studies Centre said Ukraine had not yet had the capability to hit ballistic missiles.
"When you think about a ballistic missile being fired, they come in very steep and are very fast," he said.
"So you need missiles that are much faster and much better radars in order to intercept ballistic missiles compared to cruise missiles or aircraft."
Professor Fruehling said not only was the Patriot the most sophisticated weapon system the West would provide to Ukraine, it also required the most training.
"In that sense, it's a significant step as well," he told the ABC.
US forces are training Ukrainian troops on a number of systems in other European countries.
The Pentagon announced this week that it would expand wartime training in Germany, with the aim to train about 500 Ukrainians a month from January.
The US has not indicated whether Patriot training, which can take months, has already begun.
More robust weapons systems to come?
Like most air-defence systems, several are required to protect vast areas.
With one Patriot battery being provided, what Ukraine can defend will be limited.
"You can defend the centre of Kyiv and national centres of government with it against ballistic missile attacks, but not a lot of the other towns that have been under attack with ballistic missiles," Professor Fruehling said.
"So, in that sense, it's not a game changer in any way — unless this is just the first of several announcements that might follow."
The Patriot was among several Western weapons systems, including tanks and aircraft, that Ukraine had been "keen on" since the beginning of the war, Professor Fruehling said.
There has been reluctance from the US and NATO countries to dip further into their own limited advanced-weapons supplies, but the Patriot delivery may indicate a turning point.
"What people will be looking out for is whether this now means that the reluctance to supply Western aircraft, or Western combat tanks, will also be significantly decreased," Professor Fruehling said.
"If the US is also willing to provide F16s (fighter aircraft), Abrams tanks and so on, then obviously that will have a significant impact on the war."
No Western-designed battle tanks have been supplied to Ukraine yet, but the US has indicated it would have no objection to Germany providing Leopard 2 tanks.
The Leopard 2 and Abrams are among the world's most advanced battle tanks.
"We could well see a supply of European design tanks — probably Leopard 2 designs that NATO countries have in storage — in the coming weeks if this is a broader shift," Professor Fruehling said.
'The impact is primarily on the morale'
Mr Erath said it might be too early to speculate on future weapons supply, but it was clear the US was trying to meet immediate needs and break down Russia's air-strike strategy.
"I think decisions about assistance going forward are going to be made on a case-by-case basis," he said.
He said Russia had "weaponised winter" with its recent attacks on Ukrainian infrastructure, and was trying to demonstrate to civilians that the cost of resistance was too high to continue.
"Since the initial failure of their effort to topple the government of Ukraine, their strategy has been very consistent," he said.
"They're trying to force an end to the war on their terms, meaning someone comes and says, 'What do we need to offer you to get the fighting to stop?'"
But Ukrainians have shown their willingness to resist, and the air-defence boost from the US will make it harder for Russia to keep up the current momentum.
The Patriot will raise what it costs Russia to take out infrastructure targets. It may have to send four planes to drop bombs rather than two — which in itself is "potentially game changing", Mr Erath said.
"But the impact is primarily on the morale … It's also a visible symbol of support to Ukraine from its foreign supporters."
The Kremlin has said the Patriot missile defence systems would be a legitimate target for Russian strikes against Ukraine.
Russian Foreign Ministry spokeswoman Maria Zakharova said the US had "effectively become a party" to the war by providing Ukraine with weapons and training its troops.
She added that providing Patriots would be "another provocative move by the US" and broaden its involvement in the hostilities, "entailing possible consequences".
"Any weapons systems supplied to Ukraine, including the Patriot, along with the personnel servicing them, have been and will remain legitimate priority targets for the Russian armed forces," Ms Zakharova said.
However, Mr Erath said those kinds of threats were common rhetoric.
"They don't want to have that [air-strike capability] undermined so they are trying to raise the stakes and say, 'Well, this could be escalatory, this could invite some further widening of the war,'" he said.
"Although it is difficult at this point to imagine, short of nuclear weapons, what more Russia could do to Ukraine to escalate at this point?"
