Already before the interruption of diplomatic contacts between the Soviet Union and the Polish Exile Government in April 1943 foundations of a future Polish pro-Soviet government were laid. In May 1943, the Polish Tadeusz Kosciuszko Division was formed under the Soviet patronage, and also a Union of Polish Patriots was established in the Soviet Union to represent the pro-Soviet Poles, which in fact was a tool in the Communist hands. Logically, the Czechoslovak representatives in Moscow came more and more frequently into contact with representatives of the Polish Communists and pro-Soviet oriented Poles. In addition, it became soon clear that the Polish Exile Government was getting in increasing isolation and that a new partner of Czechoslovakia would be the Polish government created under the Soviet patronage. Polish Communists, however, had to reckon with a resistance of the Polish population, which was mostly reluctant to accept the Communist ideas. Therefore, they adopted a program aimed at creating a Poland embracing in its territory all the Polish population. That is also why the new Polish government wanted to get back the Tesin area with mostly Polish population, and refused to unequivocally recognize the prewar borders of Czechoslovakia in that area. This, however, was a condition required by the Czechoslovak government to establish diplomatic contacts with the Polish Provisional Government in Warsaw. Based on many domestic and foreign sources unknown up to now the contacts between the Czechoslovak exile government and the Polish National Liberation Committee are analyzed by the author and a number of new facts are shown in relation to the diplomatic background of the recognition of the Polish Provisional Government by Czechoslovakia. The roots of the territorial conflict that broke out between Poland and Czechoslovakia soon after the end of World War II are explained.