Polish President Karol Nawrocki stripped Ukrainian President Volodymyr Zelensky of Poland’s highest honor after Ukraine named a military unit after the Ukrainian Insurgent Army (UPA), a controversial World War II-era nationalist group that engaged in the mass murder of Poles.

Poland emerged as one of Ukraine’s closest allies after the Russian invasion in February 2022, providing Kyiv with some of its strongest military and diplomatic support. As the war has dragged on, however, cracks in the relationship have grown, especially as historical areas of contention reemerge, particularly the two’s contentious relationship during World War II. These tensions flared to their worst level since the invasion this week, when Nawrocki denounced and stripped Zelensky of the Order of the White Eagle, awarded to him in 2023, over the honoring of the UPA.