The Ukrainian hero is Stepan Bandera, and the relationship in question is Ukraine's partnership with Poland. In Ukraine, Bandera is mainly remembered as the founder of nationalist groups that fought the Soviet army in western Ukraine from World War II to the 1950s, when he was in exile and later assassinated by the KGB in Germany, Ian Bateson wrote in an article titled "Ukraine and Poland's history wars are a gift for Putin" published January 24.

After Russian propaganda sought to dismiss the Maidan revolution that ousted former President Viktor Yanukovych as an exclusively far-right protest, Ukrainians began using Bandera as a symbol to troll the Kremlin, and his popularity in the country has continued to rise to new heights. On New Year's Day, thousands of Ukrainians marched in cities across the country to mark Bandera's birthday. In the Ukrainian capital alone, over 2,000 people participated in the march, carrying torches through the center of Kyiv while chanting, "Bandera is our prophet."

The problem is that in neighboring Poland, Bandera is reviled on the same level as Adolf Hitler and his inner circle. Poles across political lines mainly remember him for collaborating with the Nazis and for his followers slaughtering Polish civilians. That conviction has heightened in recent years as the country has devoted increased attention to atrocities committed against Poles during World War II.

That has forced Ukrainian and Polish diplomats to scramble in recent months to avoid a history-fueled diplomatic meltdown. On December 2, Ukrainian President Petro Poroshenko embarked on a hasty whirlwind tour of Warsaw to reassure Polish President Andrzej Duda, Prime Minister Beata Szydlo, and other senior officials that Ukraine was still committed to working together to counterbalance Russian influence in Eastern Europe and minimize conflicts over history. But, amid rising nationalism on both sides, it's not yet clear whether those efforts will suffice.

With most European powers sidelined by their own domestic tumults, the conflict with Poland could not come at a worse time for Ukraine. The United Kingdom is still consumed by the fallout from the vote to leave the European Union, France has a lame-duck president who is expected to be replaced by a pro-Russian president, and Germany is in an election year where Chancellor Angela Merkel is focused on the fallout from the migrant crisis and the terrorist attack on a Berlin Christmas market.

Meanwhile, under President Donald Trump, the United States is expected to shift from backing Ukraine to chasing a rapprochement with Moscow that could leave Kyiv out in the cold. During the campaign, Trump lavished praise on Russian President Vladimir Putin and said he'd consider recognizing Russia's annexation of Crimea. More recently, Trump has said he would be willing to end U.S. sanctions on Russia – imposed over Russia's annexation of Crimea and military action in eastern Ukraine – in exchange for a nuclear-arms reduction deal. Trump's nomination of Rex Tillerson as secretary of state has further raised concerns in Ukraine about the Trump administration. The ExxonMobil CEO was awarded the Russian Order of Friendship by Putin in 2013.

This changing political landscape has left Warsaw as Kyiv's most reliable advocate within both the EU and NATO and the only guarantor that Ukraine will stay on the international agenda. Poles across all political lines view Russian influence in Eastern Europe as a detriment to their country's security and stability. Warsaw has long made it a mainstay of its foreign policy, and the EU's, to pull post-Soviet states out of Moscow's orbit.

Since the Maidan protests, Polish-Ukrainian relations have been especially close, with both governments putting aside past disputes in order to provide a common front against Moscow. After fighting broke out in eastern Ukraine, Poland became the third-largest provider of nonlethal military aid to its eastern neighbor, in addition to dispensing much-needed humanitarian and financial assistance.

But, while both governments remain united by their opposition to Moscow, nationalist populism is playing an increasingly disruptive role in the two countries. Since the Law and Justice Party took power in 2015, the Polish government has officially named Jesus Christ as its official king and given official credence to a conspiracy theory that the 2010 plane crash that killed 96 people, including former President Lech Kaczynski, was orchestrated by Moscow.

Meanwhile, in Ukraine, Poroshenko has flirted with rising nationalism in his country to boost his lagging popular support. The president signed a bill into law that technically makes it a crime to deny or disrespect the role of World War II-era nationalist groups in fighting for Ukrainian independence as part of a package of de-communization measures in April 2015.

Read alsoPoland, Russia, Hungary among Ukrainians' popular destinations – border agencyThe Ukrainian laws were enacted by Parliament on the day that then-Polish President Bronislaw Komorowski addressed the legislature and exchanged pleasantries with Ukrainian lawmakers, shocking his Polish delegation. "It was very difficult for Poles to understand after Poland became one of the most important supporters of the Ukrainian revolution," said Lukasz Jasina, an analyst at the Polish Institute of International Affairs in Warsaw, referring to the glorification of Ukrainians involved in killing Poles.

In July, Poland's Parliament fired back when both houses approved declarations for the first time enshrining the killing of tens of thousands of Polish civilians by Bandera's Organization of Ukrainian Nationalists in the Volhynia region as an official genocide. Many observers believed the Polish Parliament's declaration showed discontent with Kyiv over the recent honoring of nationalists like Bandera. Earlier that month, as part of the de-communization process, Kyiv's City Council renamed the capital's central Moscow Prospect as Bandera Prospect.

Read alsoUkraine's Bykivnia WWII graves vandalized, Foreign Minister reacts immediatelyUkrainian responses have often failed to account for Polish sensitivities over the issue. Volodymyr Viatrovych, head of Ukraine's Institute of National Memory, has been a major advocate of expanding the role of World War II-era nationalists in Ukraine's historical memory and argued that Polish casualties in Volhynia were far lower than Polish historians claim. At a conference in September, he said Poles should consider the old name of the street, Moscow Prospect, to be more anti-Polish than the new one because of the link to Russia.

That perspective has caused concern among politicians and experts that Ukraine does not realize the extent to which it is alienating Poland through its history policies.

"The memory policies Ukraine is pursuing undermine this Polish-Ukrainian unity," said Andreas Umland, fellow at the Institute for Euro-Atlantic Cooperation, a Kyiv think tank. According to Umland, Kyiv cannot afford to risk alienating Poland, given the growing ambivalence toward Ukraine by the rest of Europe and the United States. A lack of military alliances had already allowed Russia to seize Crimea and mobilize the ongoing proxy war in the east.

And yet, in the aftermath of the Polish vote declaring the Volhynia killings a genocide, Ukrainian lawmakers continued to escalate the conflict over history. In late July, a group of Parliament members from western Ukraine introduced a bill declaring the actions of the Polish state against Ukrainians from 1919 to 1951 a genocide. After World War I, what is now western Ukraine was briefly independent before being defeated by the Polish army and absorbed into Poland. In the aftermath of World War II, Ukrainian nationalist groups continued to fight against the Polish and Soviet armies. The Ukrainian communities on the border of Ukraine were ethnically cleansed by the Polish state, first being relocated to Soviet Ukraine and later forcibly relocated to territory transferred from Germany to Poland.

"The last two presidents of Poland and the last two presidents of Ukraine agreed not to bring up this topic in the political arena," said Oleh Musiy, a member of the Ukrainian Parliament and one of the authors of the bill. "But the Polish Senate and Sejm [the lower house of Polish Parliament] decided to violate that agreement." Musiy also sent a letter to the Ukrainian foreign ministry, calling for a Polish consul in western Ukraine to be expelled after he criticized Bandera and Ukrainian historical policies in a session of the Polish Parliament.

So far the bill has yet to come to a vote and the consul remains in his post, but tensions continue to grow. A new Polish film titled "Volhynia" has stirred controversy in Ukraine by depicting Ukrainians killing Polish civilians. In Kyiv, the Polish cultural institute, part of the Polish ministry of foreign affairs, invited high-ranking Ukrainian officials to view the film, but the screening did not take place because the Ukrainian foreign ministry sent a letter to the Polish Embassy in Kyiv requesting the screening be delayed. These events have fanned rising anti-Ukrainian sentiment in Poland, with recent far-right marchers in Warsaw and eastern Poland burning Ukrainian flags and even calling for violence against Ukrainians.

Read alsoRada adopts joint Declaration of Remembrance and Solidarity, later to be endorsed by Poland’s SejmIn response, Ukraine's and Poland's foreign ministers have supported a joint investigation of the Volhynia tragedy that would result in a common version of history along the lines of Polish-German reconciliation. The reconciliation focused on addressing the most problematic moments of World War II history, such as the Holocaust and Nazi collaborations, and included developing a textbook to be used in both countries, which has become a model for the region. But Polish Foreign Minister Witold Waszczykowski stated that any official Ukrainian celebration for the upcoming 75th anniversary of the Ukrainian Insurgent Army, founded by Bandera followers, would scuttle any reconciliation.

Meanwhile, some in Ukraine have argued for letting go of Bandera as a national icon. In a popular post on the Ukrainian news website Ukrayinska Pravda titled "Hero Not of Our Time," commentator Mykhailo Dubyniansky argued that in modern Ukraine, Bandera has been completely reduced to a propagandistic cliche aimed at Moscow. "Each of us has the right to decide whether the figures of the past deserve admiration or censure," Dubyniansky wrote. "But in any case, they have earned the right to be themselves – rather than faceless projections of our current thoughts and feelings."