EXPLAINER: Why are Poland and Ukraine at odds about their history?

Poland is weighing whether to strip Ukrainian President Volodymyr Zelenskiy of its top honour for renaming an army unit after Ukrainian nationalist insurgents who massacred Poles in World War Two.

The decision caused a chorus of outrage in Warsaw and led President Karol Nawrocki to call for Zelenskiy to be stripped of the Order of the White Eagle, which he was awarded in 2023.

Here's how Poland's and Ukraine's rival interpretations of their complex, shared 20th-century history weigh on current relations:

UKRAINIAN INSURGENT ARMY

Zelenskiy signed a decree recognising a Ukrainian combat unit's contribution to the fight against Russia by naming it after the Ukrainian Insurgent Army (UPA).

During and after World War Two, when Ukraine belonged to the Soviet Union, the UPA fought against the Red Army, for a time allying itself with the Nazi German invaders, to seek Ukrainian independence.

Ukraine says the naming of the unit carries no "anti-Polish intent" and was chosen by soldiers who wanted to commemorate others who had fought against Moscow.

But the UPA was also involved in the Volhynia massacres carried out by Ukrainian nationalists from 1943 to 1945, in which Warsaw says around 100,000 ethnic Poles were killed. Thousands of Ukrainians also died in reprisal killings.

Polish historians view the massacres as a genocide intended to prevent a post-war Polish state claiming sovereignty over Ukrainian-majority areas that had been part of Poland between the two world wars.

Kyiv rejects the term, saying that thousands of Ukrainians were also killed in what was a complex conflict.

The events have been a bone of contention for decades, even as Poland has strongly backed Ukraine in its fight against Russia's invasion, taking in almost a million refugees and supplying weapons.

In 1947, within new borders after World War Two, Poland forcibly relocated some 140,000 ethnic Ukrainians and people identifying as members of the small Lemko ethnic group from southeastern Poland to territories it had regained from Germany. The aim was to cut support for underground UPA groups in Poland, but the Ukrainian side considers it a crime of ethnic cleansing.

DEMANDS FOR EXHUMATIONS IN UKRAINESuccessive Polish governments have, with limited success, demanded access to the sites in western Ukraine that were once part of Poland where UPA massacres took place.

However, last year Poland began exhuming the remains of Poles killed in the former Polish village of Puzhnyky and, last week, Kyiv gave permission for more exhumations in Volhynia's Liuboml district.

NATIONALIST PRESIDENT NAWROCKI

Nawrocki, a conservative nationalist historian inspired by U.S. President Donald Trump, has repeatedly accused Kyiv of stalling on requests for exhumations, and urged it to denounce the Volhynia massacre as genocide.

Nawrocki has tapped into weariness with the large number of Ukrainians in Poland and, during his campaign, vowed not to ratify any Ukrainian accession to NATO to avoid provoking Russia, departing from previous Polish policy and angering Kyiv.

Critics have accused Nawrocki of promoting an approach to history teaching that whitewashes difficult parts of Poland's past.

-Reporting by Marek Strzelecki; additional reporting by Anna Wlodarczak-Semczuk; Editing by Kevin Liffey/Reuters