The Volhynia massacre in 1943 is the subject that divides Poland and Ukraine. Nearly 100,000 Poles were killed by Ukraine's insurgent army.
Poland called the massacre genocide in 2016, but Ukraine did not. Poland is pushing to see its Ukrainian ally integrate the European Union as soon as possible. But Poland knows that some problems could resurface if it were to be integrated into the common market. This is also why Poland is raising sensitive issues, which Ukraine does not prefer to address for the moment.