Nineteen retired US generals and former officials have called on the Biden administration to step up the pace of arms supplies to Ukraine or run the risk of "unintentionally seizing defeat from the jaws of victory".
They said that the US was providing enough weaponry to ensure a stalemate but not sufficient to help Ukraine recapture territory seized by Russia. The former officers, diplomats and other officials argue the administration is inhibited by fear of triggering a Russian escalation, possibly involving nuclear weapons - but they argue that failure to defeat Vladimir Putin in Ukraine increases the danger of a confrontation with Moscow later "on less favourable grounds".
The pace of arms deliveries to Ukraine has been a repeated source of friction between Washington and Kyiv, as well as some eastern European allies. So far, the US has given Ukraine nearly $10bn in military aid under the Biden administration, including rocket launchers, but has stopped short of providing longer-range missiles, fixed-wing aircraft and certain armed drones.
"By providing aid sufficient to produce a stalemate, but not enough to roll back Russian territorial gains, the Biden administration may be unintentionally seizing defeat from the jaws of victory," a commentary published in the Hill said, signed by Gen Philip Breedlove, former supreme commander of Nato forces in Europe, and three former ambassadors to Ukraine, Marie Yovanovitch, John Herbst and William Taylor, among others.
"Out of an over-abundance of caution about provoking Russian escalation (conventional as well as nuclear), we are in effect ceding the initiative to Russian President Vladimir Putin and reducing the pressure on Moscow to halt its aggression and get serious about negotiations," the commentary said.
"We may think that each day we delay providing Ukraine the weapons it needs to win, we are avoiding a confrontation with the Kremlin. To the contrary, we are merely increasing the probability that we will face that danger on less favorable grounds."
Last month, Joe Biden's national security adviser, Jake Sullivan, said that the pace of arms deliveries was limited by Ukraine's capacity to absorb them in terms of people trained to use them, and the need to have Nato allies support the deliveries.
Talking about the Army Tactical Missile System (ATACMS) which have a 300km range (186 miles) and which Kyiv was repeatedly requested, Sullivan said that fear of escalation played a role in withholding them.
"[A] key goal is to ensure that we do not end up in a circumstance where we're heading down the road towards a third world war," he said at the Aspen Security Forum.
The signatories on the commentary argue that the US is allowing itself to be deterred by the Russian nuclear threat while not taking into account how much the US arsenal deters Russia.
"I've never seen the United States deterred from doing what's in its interest, but Russia aggression as we are seeing now," Herbst, now a senior director of the Atlantic Council's Eurasia Center, said. "I think it is the most important reason for what I call the timidity and the slowness of the administration's response to legitimate and important requests, even urgent requests."
Alongside ATACMS, the supply of fixed-wing aircraft has been a source of contention between Washington, Kyiv and eastern European capitals. Some European countries have supplied spare parts to Ukraine that has enabled Kyiv to repair some Soviet-era MiG jets, but the US and some allies view the supply of whole aircraft as escalatory.
In March, Poland announced it was prepared to hand over its 28 MiG-29 fighters to the US base in Ramstein, Germany, leaving it to the US or Nato as a whole to hand them over to Ukrainian pilots, but the suggestion was rejected by the Pentagon.
Slovakia was reported to be prepared to send its MiG-29s, but the country's defence minister, Jaroslav Naď, said over the weekend that they would remain in service until the end of August. ď
"We are negotiating with our allies and partners regarding what to do with them next. No decision has been made yet," Naď said on Facebook on Sunday.
Debra Cagan, a former US deputy assistant secretary for coalition operations and another signatory of the Hill commentary, said she believed Washington had yet to approve the transfer.
"There's no way shipments of aircraft would ever happen to Ukraine without the US giving the go-ahead," Cagan, now a fellow at the Transatlantic Leadership Network, said.
"The Slovaks are not in a position militarily to just say: ok we're going to do this and arrange for Ukrainian pilots and all this … Someone from the White House has to say you need to make this happen. My understanding is that has not occurred."