Poland says it’s taking a stand over a bloody massacre. Ukraine says it’s a needless affront to a nation fighting for survival.

Limited time: Save 25% on NBC News subscription

Get exclusive reporting, live Q&As and ad-free reading.

Kyiv and its allies may be feeling pretty good about the present state of the war with Russia, but an intense historical feud has reignited, threatening relations between Ukraine and one of its most ardent backers.

The dispute between Warsaw and Kyiv comes just as Europe has re-engaged the United States in pressuring Moscow to end the war, with officials on the continent warning the neighbors’ tensions would play into the hands of President Vladimir Putin.

President Karol Nawrocki’s move to strip Ukrainian President Volodymyr Zelenskyy of Poland’s top honor earlier this month elevated the spat to a full-blown crisis, with Zelenskyy skipping a key wartime conference in Poland that began Thursday.

It all centers on the Ukrainian Insurgent Army (UPA), which Poland accuses of mass killings during World War II.

Some in Ukraine view the UPA as nationalist heroes for their resistance to Soviet and Nazi forces. But Zelenskyy’s decision to name a military unit after the UPA drew fury from Warsaw, which had already shown signs of war fatigue over the influx of refugees and economic drain of supporting Ukraine’s fight.

The timing of the dispute risks marginalizing Poland and shutting it out of the Ukraine peace process, said Ukrainian lawmaker Mykola Kniazhytskyi, who co-heads the Ukraine-Poland Interparliamentary Group and spoke with NBC News on the phone from this week’s conference in Gdansk on postwar reconstruction.

“Obviously, there is nothing good about this,” he said.

The decision to revoke Zelenskyy’s honor was not directed against the Ukrainian people and Poland’s support will continue, Nawrocki said in a lengthy statement last week, “because we know that Russian aggression poses a threat to the security of Poland and all of Europe.”

Poles opened their homes to millions of Ukrainian refugees escaping Russia’s onslaught in early 2022, and the NATO member has remained steadfast in its military support for Ukraine and its deeper integration into the European community. Still, according to Nawrocki, Ukraine’s path toward E.U. membership requires a “willingness to honestly confront the difficult chapters of its own history.”

Poland’s right-wing opposition leader went a step further, urging his country’s government to block Ukraine’s accession to the European Union, weeks after the first phase of membership talks received the green light.

Warsaw is a crucial logistical hub for weapons flowing into Ukraine from the West and an important mediator in its dealings with Europe. A shift away from this position “will lead to a lot of problems for Ukraine,” Kniazhytskyi said.

The two sides accuse each other of using the issue to score domestic political points: Polish officials say Zelenskyy wants to distract from corruption scandals, while the Ukrainian leader has suggested his counterpart in Warsaw is focused on elections next year.

But Nawrocki insists he cannot put aside historical injustice because it may be inconvenient now.

“Facts are not subject to negotiation; they do not change with political circumstances or necessities,” Nawrocki said.

Poland says the UPA’s members massacred around 100,000 Polish villagers in the Volhynia region, now a part of northwestern Ukraine, between 1943 and 1945 in what it recognizes as genocide. Thousands of Ukrainians also died in reprisal killings.

Describing the UPA’s actions in Volhynia as “very brutal ethnic cleansing,” historian Zbigniew Wojnowski at the University of Oxford said the perception of the group in modern Ukraine is grounded in what happened after that — the prolonged and painful struggle against the Soviets that followed World War II and stretched into 1950s. “It’s highlighted in Ukrainian public memory as a way of boosting morale in the current war against Russia,” said Wojnowski, who specializes in Ukrainian and Soviet history.

The Ukrainian Institute of National Remembrance, a body coordinated by Ukraine’s prime minister, does not refer to the killings as massacres, calling them a “tragic page” in the history of both people and part of the “Ukrainian-Polish confrontation.”

The conflicting interpretations are part of the “unprocessed trauma” of the massacres for both Poles and Ukrainians, Wojnowski said. Despite ongoing exhumation work to honor the victims, the full scale and political context that preceded the killings are not well understood on either side, he added, helping to breed these persistent divisions.

Asked if Ukraine and Poland are “not friends anymore,” Zelenskyy told Ukrainian TV earlier this week: “Ukraine and Poland cannot be anything but partners and friends because we are neighbors.” Ukraine remains open about its history, Zelenskyy said, adding that Ukrainian soldiers were protecting Poland against Russia right now, not the other way around.

Zelenskyy said it was the soldiers who asked him to rename their unit after the “heroes of UPA,” but that he supported them fully. The day after Nawrocki announced his decision to withdraw the Polish honor, Zelenskyy shared photos of the blue-ribboned award, featuring an encrusted eagle, being shipped back to Poland.