Harris will land in Poland on a mission to rally NATO against Russian aggression. But her visit could now be dominated by the question of why the U.S. and Poland have fumbled the transfer deal.

On a call previewing the trip, a senior administration official conceded the issue would be front and center. “We have been in dialogue with the Poles for some time about how best to provide a variety of security assistance to Ukraine,” the official said. “And that’s a dialogue that absolutely will continue up to and as part of the vice president’s trip.”

— Schedule highlights: On Thursday, Harris is holding a bilateral meeting with Polish President ANDRZEJ DUDA and PM MATEUSZ MORAWIECKI, and a separate meeting with Canadian PM JUSTIN TRUDEAU, who is also in Warsaw. She’ll also meet with some of the thousands of Ukrainian refugees in the country. In Romania on Friday, Harris will meet with President KLAUS IOHANNIS.

It is rare that a VP steps into the middle of such an important diplomatic negotiation. Ukraine is desperate for air power. The Poles have kicked the decision to the Americans (or tried to). The question of whether a transfer can happen without triggering a dangerous escalation is unanswered. The world will be watching how Harris handles this delicate moment. If she unlocks a U.S.-Poland-Ukraine transfer deal, it would be a monumental foreign policy success for someone who has few to speak of in her time as a senator and VP.