Diplomatic relations between Ukraine and Poland haven’t been so tense in decades.

The fallout began after Ukrainian president Volodomyr Zelensky signed a decree last month naming an elite military unit, the Sever Special Operations Centre, after “the heroes of the Ukrainian Insurgent Army,” the UPA.

Since then, leaders of both nations have been returning their respective country’s medals.

In Ukraine, the UPA is seen as a symbol of heroic resistance against Soviet forces in the fight for independence.

In Poland, it is notorious for killing up to 100,000 Poles in the Volhynia and Galicia regions between 1943 and 1945, in an effort to ensure the territory did not become part of postwar Poland.