Around two years after Ukraine started asking allies for F-16 fighter jets to help it fight Russian forces, the first planes are set to arrive by next month.
The length of the process, from procuring the U.S.-designed aircraft and training Ukrainian pilots to fly them, has frustrated Kyiv.
Russia has had time to prepare defences to try to nullify the F-16s' impact, and Ukraine has had to survive with a depleted air force a fraction of the size and sophistication of the enemy's.
Here are some facts about how the F-16s may help Ukraine and what obstacles still lie in the way of effective deployment:
POTENTIAL IMPACT
Some analysts say the F-16s will not alone prove a turning point in the war, which began with Russia's full-scale invasion in February 2022.
"You'd have to separate symbolism from the actual impact on the battlefield - which will be useful but modest, particularly in the beginning," said Mark Cancian, senior adviser with the International Security Program at the Center for Strategic and International Studies (CSIS).
Serhii Kuzan, chairman of the Ukrainian Security and Cooperation Center, a non-governmental research group, said at least 60 planes would be needed for significant operations as Ukraine attempts to push Russian aviation back from its borders.
Lawmaker Oleksandra Ustinova, who leads Kyiv's parliamentary commission on arms and munitions, said that Ukraine would need nearer to 120 F-16s to boost its air capability significantly.
While the pilots gain experience in Ukrainian skies and the military builds out its air infrastructure, the initial deliveries could at least help Ukraine strengthen its air shield, experts say.
"It will provide some air defence and depth capacity, potentially also help intercepting Shaheds [Iranian-built drones] and cruise missiles. Although it is a very expensive way of doing that, munitions-wise," said Justin Bronk, senior research fellow for airpower and technology at Royal United Services Institute (RUSI).
Ukraine's military has worked hard to reduce the threat to the arriving F-16s in recent months by attacking Russian air defences, according to Kuzan.
"The formation of the battlefield, especially in the south, is already taking place," he said. "Ukraine has the capabilities to systematically strike Russia's foremost air defence complexes."
But Cancian of CSIS said he expected Ukraine to try to open gaps in Russia's defences in the immediate run-up to planned F-16 attacks rather than a long time in advance.
PILOTS AND MAINTENANCE
Training will be crucial.
"You can have lots of fast jets but if they don't have effective weapons, and air crew able to employ them with effective tactics, then they will just be shot down in large numbers," said Bronk.
The timeline for the training of Ukrainian pilots on F-16s has dominated discussions about deliveries and pledges of more than 70 jets.
By the end of 2024, Ukraine expects to have at least 20 pilots ready to fly F-16s, Ustinova said.
"It is difficult to solicit more planes when you don't have people to pilot them," she said, adding that, at first, Ukraine will have more F-16s than qualified pilots.
"Waiting in line for 10 years before our pilots are trained is not OK."
The Ukrainian Air Force spokesperson declined to comment.
U.S. officials have directed questions on training to Ukraine and noted that pilots can also be trained in Europe. However, Bronk said NATO's capacity was already stretched.
He added that aircraft maintenance was an even more pressing challenge than pilot training.
He said most repairs and maintenance would need to happen inside Ukraine, and it would probably have to rely on foreign contractors who know the aircraft.
AIR BASES UNDER THREAT
Russia has already intensified its attacks on infrastructure that could be used for the maintenance and deployment of F-16s, some experts said.
"Russia is striking all airfields, potential F-16 bases, every day, including attempts to damage airstrips and infrastructure. These strikes have not paused for the last two months, at least," Kuzan said.
The targets will become all the more valuable when the aircraft, pilots and maintenance teams arrive. This is likely to force Ukraine to install missile defences to protect them, even though it is short of both air defence systems and ammunition.
"We have to accept the fact that the airfields will be well-protected when civilian objects could be under attack," Kuzan said, adding that each base would need at least two Patriot and two NASAMS batteries to secure it.
"As soon as we (build up our flight capabilities), we will push their planes back and the terror will stop. But these couple of months will be truly difficult," Kuzan added.
