The article is a continuation of the study on Polish–Czechoslovak relations in the months after the Provisional Government of the Republic of Poland was formulated. It concerns the dramatic weeks from the end of World War II in Europe until the completion of the so-called Yalta Formula and the establishment of the Provisional Government of National Unity. During that period, the border conflict escalated to cover the issue of the post-war affiliation of the Kłodzko Valley, the Opole region and Lower Silesia. Trying to authenticate themselves to their societies, the governments of both countries pursued an aggressive policy in the matter of mutual delimitation, at times on the verge of open armed conflict. Both sought Soviet support for their territorial demands, trying to give the impression that they did not perceive Moscow’s cynical game, which provoked and then used the situation to make them even more dependent on each other. The article ends with the June round of Polish–Czechoslovak negotiations in Moscow, which were broken without any agreement on the key issue of the border.