UKRAINIANS call it a tragedy, for Poles it was a massacre. Between February 1943 and February 1944, units of the Ukrainian Insurgent Army killed up to 100,000 Poles in Volyn and eastern Galicia, former Polish territories now in western Ukraine. The butchery reached its apogee in July, with as many as 20,000, including women, children and the elderly, murdered. Around 20,000 Ukrainians also died at the hands of Poles or Ukrainians who saw them as too close to the hated occupiers. It remains one of the darkest chapters in the two nations' histories, and one of the most misunderstood. Many Poles have an idealistic view of their Kresy, the eastern borderlands to which Volyn belonged, as a collection of quaint, provincial towns and villages where Poles, Jews and Ukrainians lived in harmony, explains Andrzej Szeptycki, an expert in bilateral relations from the Warsaw University. The axes and pitchforks of Ukrainian nationalists, they believe, brought an end to that idyll.

Reality was more complex. Throughout the interwar period, Poland practiced a harsh policy of assimilation of its national minorities, particularly Belarusian and Ukrainians, fearing they would become a fifth column. In addition to trampling cultural and religious rights, land was seized and redistributed to Polish military veterans, in hopes of reigning in the east.

Among those regions Volyn stood out, explains Yaroslav Hrytsak, a Lviv-based historian. A former Russian gubernia, lacking the parliamentary and intellectual traditions of its ex-Habsburg neighbours, Volyn saw a unique experiment in nation building. Henryk Józewski, a former Polish spy it in Soviet Ukraine, was picked by Poland's interwar strongman Józef Piłsudski (pictured) to build a Ukrainian nation that is loyal to Poland, notably by supporting cultural initiatives and creating a loyalist Ukrainian party.

Despite its relative success, the programme was reversed after Piłsudski's death in 1935. Ukrainians were barred from government jobs, workers' protests suppressed; Orthodox churches were destroyed and their adherents forcibly converted to Catholicism. As a result, both communist and nationalist elements among the Ukrainians called for ethnic cleansing of Poles from Volyn, even though the Soviet mass starvation, known as the Holodomor, or death by hunger, had just cost millions of Ukrainian lives across the border.

“It was worse than a crime, it was a stupidity,” Mr Hrytsak says. “You had a terrain that was ready to explode.” What followed was genocide, he says, but it must be viewed within the context of a pyramid of genocidal acts committed on the territory of Ukraine in the 1930s and 1940s, starting from the Holocaust and ending on Akcja Wisła, the forced deportation of tens of thousands of Ukrainians.

The term arouses heated debate. Last week, Poland's parliament narrowly voted down an amendment to describe the events in Volyn as genocide. Across the border, 148 members of the ruling Party of Regions and their Communist allies appealed for Poles to pass it, hoping to stir up bad blood with, Svoboda, a right-wing party. Svoboda asked Bronisław Komorowski, the Polish president, to cancel a visit on July 14th to Lutsk, the region's capital, for a 70th anniversary memorial service.

Barring an egg thrown at Mr Komorowski, the event was quite peaceful, but also much more subdued compared to previous commemorations. Viktor Yanukovych, the Ukrainian president, was conspicuously absent. His presence would have made it harder to keep the nationalists at bay, said a relieved Ukrainian diplomat.

Relations between the two neighbors have grown edgy. Poland, Ukraine's principal advocate in Europe, is frustrated at Kyiv’s lack of progress on implementing pro-European reforms and keeping Yulia Tymoshenko, the former prime minister, behind bars. Diplomats complain Ukrainian authorities ignore appeals to protect Polish firms from corporate raiding. Some believe relations with Russia should be prioritised: the potential perks are bigger, and Ukraine might start treating them seriously.

With the right-wing on the rise in both countries, things are unlikely to get easier. Svoboda, which first entered Ukraine’s parliament last October, prefers autonomy to integration. In Poland the main opposition party, Law and Justice, which leads recent polls, is cosying up to the anti-Ukrainian elements of the so-called Kresy group, warns Mr Szeptycki. The party voted to include the mention of genocide, despite appeals by Radosław Sikorski, the foreign minister, “not to denigrate Ukraine”.

“For several years now some people feel the issue of Volyn is being sacrificed in the name of better relations with Ukraine,” Paweł Kowal, who chairs the EU-Ukraine parliamentary cooperation committee, explains the heated rhetoric. He says Mr Komorowski has deftly managed his role as a mediator, but overcoming historical problems will require years of sustained efforts, as was the case with Germany.

But time is limited. Ukraine still hopes to sign an Association Agreement with the EU in Vilnius this November. Mr Sikorski has said it has not yet met the necessary criteria; an initial May deadline for improvement was pushed back to June and then September. With the European Parliament facing elections in 2014, and Ukraine in 2015, failure now could mean years of delays, EU leaders warn. By then Poles, too, will head to the ballot box, and reconciliation could slide even further away.