In July 2023, the presidents of Poland and Ukraine commemorated the victims of a wave of violence that left tens of thousands of people dead during World War II.

Ukraine's Volodymyr Zelenskyy and Andrzej Duda, then Poland's president, knelt side by side and placed candles in front of a memorial near the altar of a church in Lutsk, capital of the Volyn region in northwestern Ukraine. In social media posts, both sent a positive message: "Memory unites us!"

That proposition is being put to a tough test three years later, as a rekindled dispute over what Poland calls the Volhynian Massacre and Ukraine calls the Volyn Tragedy rocks relations between the neighbors, risking repercussions for Kyiv's defense against Russia and for Ukraine's EU integration.

The fuel for the flare-up was Zelenskyy's decision in May to give a military unit a name honoring the "Heroes of the UPA" -- the Ukrainian Insurgent Army, which Poland says massacred some 100,000 Poles in the area, then under German control, in 1943-45, killings Poland's parliament has designated a genocide.

Ukraine rejects that designation and says Ukrainians faced hatred from Poles as well. Thousands of Ukrainians were killed in reprisals.

The UPA was the military arm of nationalist resistance leader Stepan Bandera's Organization of Ukrainian Nationalists (OUN), fought against Nazi Germany and the Soviet Union at different times during the war. The groups are accused of carrying out murderous campaigns against Poles and Jews.

But manyUkrainians hail them as freedom fighters -- a perception that has widened as the country tries to fight off a Russian invasion 35 years after it won independence from Moscow as the Soviet Union collapsed.

Poland, steeped in its own bad memories of the Nazi-Soviet invasion in 1939 and decades of dominance by Moscow after World War II, has been one of Ukraine's strongest backers since Russia launched its full-scale invasion in February 2022.

As Zelenskyy's appearance with Duda in 2023 indicated, both countries had kept the historical tensions largely at bay. But now, the public spat is injecting poison into bilateral relations and raising the prospect of broader problems at a crucial time.

"Wading into a conflict between politicians in Poland and Ukraine ‌is a strategic mistake that will harm both sides: business-wise, geopolitically, and reputationally," Polish Prime Minister Donald Tusk warned in a post on X last month.