Ukraine’s Prime Minister announced on Tuesday, June 24th, that she will be leading Kyiv’s delegation at the Ukraine Recovery Conference (URC). This could sound routine—except for the major diplomatic controversy that has blown up between President Volodymyr Zelensky and the Polish government.

Since the decision, PM Yulia Svyrydenko has neither mentioned Zelensky directly nor given any official reason for his non-attendance. The absence was also confirmed by Polish deputy prime minister, Władysław Kosiniak-Kamysz.

Initially, Zelensky was scheduled to attend the event in Gdańsk. However, a diplomatic crisis began last month after the Ukrainian president named a military unit after the “heroes of the Ukrainian Insurgent Army (UPA).”

The UPA is celebrated in Ukraine as a force for Ukrainian independence from Moscow, but in Poland it is notorious for the Volhynia massacres—the slaughter of around 100,000 civilian ethnic Poles, mostly women and children.

In response, Zelensky was stripped of his Order of the White Eagle, awarded to him by Poland. As he returned the honour, a number of Polish public figures did the same in solidarity with the President.

Also excluded from this year’s URC, which takes place under the auspices of pro-Brussels Polish PM Donald Tusk, is President Karol Nawrocki. This is due to what Polish government spokesman Adam Szłapka calls the “format of the event.”