Washington, May 20 (AP) The US has once again buckled under pressure from European allies and Ukraine's leaders and agreed to provide more sophisticated weapons to the war effort. This time it's all about F-16 fighter jets.

Ukraine has long begged for the sophisticated fighter to give it a combat edge as it battles Russia's invasion, now in its second year. And this new plan opens the door for several nations to supply the fourth-generation aircraft and for the US to help train the pilots. President Joe Biden laid out the agreement to world leaders meeting in Hiroshima, Japan, on Friday, according to US officials.

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So far, however, the US has provided no details and said decisions on when, how many, and who will supply the F-16s will be made in the months ahead while the training is underway. Details on the training are equally elusive. US officials spoke on condition of anonymity to discuss decisions not yet made public.

Still, with this decision, the Biden administration has made a sharp reversal, after refusing to approve any transfer of the aircraft or conduct training for more than a year due to worries that it could escalate tensions with Russia. US officials also have argued against the F-16 by saying that learning to fly and logistically support such an advanced aircraft would be difficult and take months.

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Here is a look at the fighters, why the US has been reluctant to provide them to Ukraine and what is known and not known yet about the decision.

WHY DOES UKRAINE WANT F-16 FIGHTER JETS?

Ukraine has pressed for Western jets since the very earliest stages of the war, insisting that the sophisticated aircraft would give them a leg up in the war and allow them to strike Russian forces.

Nearly a year ago, two Ukrainian fighter pilots who asked to be identified by their callsigns "Moonfish" and "Juice" met with reporters in Washington to argue for getting the F-16 Fighting Falcons, which have more advanced radars, sensors and missile capabilities.

In February, Ukrainian Defense Minister Oleksiy Reznikov held up a picture of a warplane when he was asked in Brussels what military aid his country needed. And earlier this month Ukrainian President Volodymyr Zelenskyy said during a visit to Germany that he was pushing for allies to forge a "fighter jet coalition" that would provide Ukraine with the combat planes it needs to counter Russia's air dominance.

Ukraine's leaders have argued that the F-16 is far superior to their existing fleet of Soviet-era warplanes. In response to those pleas, the U.S. has found ways to deliver some of the advanced capabilities without providing the actual jets.

For example, Air Force engineers found ways to modify the HARM air-to-surface anti-radiation missile so that it could be carried and fired by Ukrainian-flown MiGs. The missile and its targeting system enable the jet to identify enemy ground radars and destroy them.

WHY HAS THE US BALKED?

Repeatedly for months senior US officials — from Biden on down — had flatly rejected sending F-16s to Ukraine, when asked publicly. And the US had so far declined to allow other countries to export their US-made Falcons to Ukraine.

As recently as Monday, after Zelenskyy reiterated his desire for F-16s and other jets, National Security Council spokesman John Kirby was asked if the U.S. had in any way changed its position on F-16s not being the right focus for military aid. Kirby said, "No."

Asked similar questions in recent months, Biden also declined to approve the F-16s. In one instance earlier this year he was asked why he opposed sending them, and he responded, "Because we should keep them here."

US officials at the Pentagon have insisted that the military aid the US was providing to Ukraine was based on what the country needed most to fight the war. So the emphasis has been on sending air defense systems and millions of rounds of rockets, missiles and other ammunition — as Ukraine prepares for a much expected spring offensive.

The other key reason, however, is the ongoing concern that sending fighter jets to Ukraine would enrage the Russians, provoke President Vladimir Putin and possibly escalate or broaden the war.

WELL, ON SECOND THOUGHT ....

Despite all the concerns, the U.S. has proven again and again during the war that it can change its mind.

Early on the U.S. balked at sending Patriot missile batteries, longer-range missiles or tanks. And in each case, it eventually succumbed to pressure from allies and agreed to send the increasingly advanced weapons.

Of note was the recent turnabout on M1A1 Abrams tanks. For months the US had said the Abrams was too complicated and required too much logistical support for Ukrainian troops. Under escalating pressure from European nations that wanted to send Ukraine their own tanks, the U.S. finally agreed to send 31 Abrams to Ukraine. Training is expected to begin soon.

The F-16 approval has been a long, slow slog. Despite public insistence — for months — that there was no movement on the F-16s, the Pentagon in March brought two Ukrainian Air Force pilots to the Morris Air National Guard Base in Tucson, Arizona, to familiarize them with the F-16 and learn how pilots are trained.

US officials refused to discuss the event publicly, but privately they said the two pilots flew F-16 simulators and got a feel for the training. The U.S. Air Force, meanwhile, got insight into how long it would take for an experienced Ukrainian fighter pilot to learn the F-16's more advanced systems. Officials determined that realistically it could be done in about four months, if the pilots were already trained to fly their own Soviet-era fighters.

WHAT WE STILL DON'T KNOW

According to US officials, Biden told leaders in Japan that the US will participate in the F-16 training, and that decisions on providing the jets will come later.

Officials said it's still not clear if the US will simply allow other nations to send F-16s to Ukraine, or if the US will also send some. And there are no estimates on how many of the jets will be provided or when. Officials acknowledge that it will not be in time for the anticipated spring offensive.

And while officials said the training will begin soon, it isn't yet clear where it will be, how many pilots will be trained and how long it will take.

The US Air Force has two F-16 air wings in Europe: the 31st Fighter Wing at the Aviano Air Base in Italy and the 52nd Fighter Wing at Spangdahlem Air Base in Germany. The US also routinely sends F-16 fighters in and out of Europe on a rotational basis in smaller groups. (AP)

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