End of The Concordat
During World War II the Holy See had appointed German and Slovak priests to Polish parishes, violating the concordat. These actions have been condemned by Polish government in exile, which considered it a betrayal and issuing statement saying "Pius XII's decision is tantamount to the acceptance of illegal German demands and comprises an unfriendly act towards the Polish people".
On 12 September 1945, the Polish Provisional Government issued declaration that concordat is no longer valid, citing these violations by the Church. This view is not shared by the Holy See and many Polish priests, such as Stefan Wyszyński, who consider concordat to have only been ended by the Polish side by this declaration.
In a larger context, not only Poland, but all East European countries under Soviet control cancelled their concordats with the Holy See after 1945, which can be seen as a part of the Persecutions against the Catholic Church during the pontificate of Pope Pius XII.
Read more about this topic: Concordat Of 1925