Even before Polish President Andrzej Duda arrived in the eastern Ukrainian city of Kharkiv on December 13, it was clear that this trip was being made under very difficult circumstances.

Poland and Ukraine are facing the biggest challenge in their bilateral relationship because of the past. Both are returning to World War II events in ways that are poisoning the special rapprochement between the two countries that began soon after 1989 but has been deteriorating since 2015.

Just before Duda’s visit, a statement on his presidential website listed Polish-Ukrainian disagreement on how to deal with the victims of World War II as one of the top priorities to be discussed with Ukrainian President Petro Poroshenko.

The statement concerned a decision taken by the Ukrainian Institute of National Remembrance in April 2017. This decision forbids the exhumation of Poles who were victims of anti-Polish Ukrainian war crimes that were carried out in the Volyn region in 1943. Ukraine took this decision in response to the destruction of a monument to soldiers of the Ukrainian Insurgent Army (UPA), the military arm of the Organization of Ukrainian Nationalists (OUN), in the Polish village of Hruszowice in April 2017.

These retaliations show how Volyn represents the unfinished rapprochement between Poland and Ukraine. In 1943, tens of thousands of Polish civilians were killed when, under Nazi occupation, the UPA tried to purge all non-Ukrainians, namely Poles, from the future Ukrainian nation-state. As the Polish Home Army (AK) responded to UPA’s attempted ethnic cleansing, the region was drawn into a bloody civil conflict that lasted until late 1944.

The Polish-Ukrainian battle for the past has consequences for the region. Russia could benefit from the break in the bilateral rapprochement. Poland had once been a staunch defender of Ukraine moving closer toward the European Union. Weakening Polish support would be a big loss for Kiev at a time when it needs more allies than ever inside the EU.

The historical events in Volyn and Eastern Galicia have always been problematic for Poland and Ukraine. But recently, their different interpretations of historical events have led to a severe deterioration of bilateral relations. Both governments and their respective Institutes for National Remembrance have fostered the renationalization of historical narratives and historiography for different reasons.

In Ukraine, the World War II nationalist and independence movement under Stepan Bandera, though controversially assessed by many Ukrainian historians, has gained importance as part of the state’s historical narrative on the background of the Ukrainian-Russian conflict.

With its emphasis on the fight for Ukrainian national independence against foreign occupation and suppression throughout World War II and beyond, this narrative plays an important role in Kiev’s self-defense against Moscow. Alas, renaming streets in honor of Stepan Bandera and making him a national hero is much more than an internal political act of an independent nation-state. Kiev is playing with fire when it comes to its relationship with Poland, which is led by the national-conservative Law and Justice Party (PiS).

Since regaining power in 2015, PiS has put a lot of effort into streamlining a national historical narrative focusing on Polish victimhood and resistance.

From the PiS point of view, history is “good” if it is “patriotic”—that is, if it emphasizes Polish heroism and losses. In spring 2017, historian Paweł Machcewicz was removed from his post as the Director of the Museum of World War II because the exhibition was not “patriotic” enough. In Poland, the historical events of 1943 in Volyn have developed into one of the core Polish lieu de mémoire.

The Polish Institute of National Remembrance has launched a special website dedicated to Volyn. In July 2016, the Polish parliament almost unanimously adopted a resolution declaring the extermination of Polish civilians by Ukrainian nationalists in Volyn a genocide. Ukrainian President Petro Poroshenko expressed his regret about this decision. Shortly after that, Volyn and memory politics linked to it entered into Polish mass culture with the release of a popular film, “Wolyn” (Hatred). Ukraine, on the other hand, so far officially refers to the events of 1943 as the “tragedy of Volyn” and refuses to use the term genocide.

The unfinished reconciliation process originates in a long-term suppression of the Volynian conflict in Soviet times.

Before 1989, in neither country had the events in Volyn been part of any public discourse or any history textbooks at school. Their commemoration was limited to private memories among families in western Ukraine and among those Poles who had been affected.

After 1989, based on the so-called Giedroyc doctrine, Poland got rid of imperial legacies and put an end to historical disputes with its eastern neighbors. Starting in the 1990s, many political and civil society initiatives tried to bridge the historical gaps between Poland and Ukraine.

For almost 20 years, reconciliation with Ukraine was an integral part of Poland’s self-perception as a free and independent nation-state. But the time span since 1989 was too limited and the number of people involved in the process on both sides was too small to heal the wounds of the past.

In 2015, against the background of the conflict with Russia, Ukraine started to emphasize its historical fight for independence; at about the same time, PiS began to stress the historical role of Poles as freedom fighters against external oppression. Two different varieties of victimhood confronted each other. Ukrainian victimhood is mainly part of Kiev’s foreign and defense policy toward Moscow. Polish victimhood is part of domestic politics aimed at securing the power of the ruling PiS. The government presents Poland as a country surrounded by enemies and thus having to be internally strong, well-fortified, and patriotic.

This dispute about different interpretations of the common past has poisoned this neighborhood relationship at time when Ukraine needs the support of Poland more than ever. But Poland is also paying a price for the tough course the government is taking in memory politics. Society is split. A recent survey by the Körber Foundation revealed that 53 percent of the population supports the government’s patriotic position on history and 43 percent oppose it.

This gap will be difficult to overcome as the atmosphere between the two groups—sometimes among members of the same family or people who had fought together for Poland’s freedom during the Solidarity movement—has become worse.

And there is another internal challenge: for the 1 million Ukrainians currently working in Poland, the growth of open anti-Ukrainian sentiment is particularly disturbing. If they turn their back on Poland, the Polish economy might take a hit.

To prevent further bilateral damage, Poland and Ukraine should leave the historical battlefields of 1943 and continue their reconciliation and mutual cooperation. In doing so, they would also prevent Moscow reaping the benefits from this poisoned neighborhood.

Gabriele Woidelko is director of the “Russia in Europe” program at the Körber Foundation in Hamburg.