This paper presents the reasons for, and the historical-political conditions underpinning, the liquidation and division of East Prussia after World War II. It is recalled here that, even before the War, Polish scholars, including quite a number of leading Polish geographers, had aimed to have such a political design implemented. The period of the War itself saw respective proposals drawn up in various detailed variants. The very first two of these were presented on April 2nd, 1940, in Paris by Division III of the Head Office of the Polish Army Commander (at that time General Władysław Sikorski). The subsequent proposal for the course of the border was as finalised by October 17th, 1940 in London, by the Bureau for Political Studies of the Polish Government-in-Exile. The three variants are presented and commented upon in the article, as well as shown in Fig. 1. They did not gain any high political status, unlike the decision on the division of East Prussia taken personally by Joseph Stalin in July1944, during a meeting with the Polish Delegation. This concept for the course of the border is also shown in Fig. 1. The defeat of Nazi Germany and the occupation of this German border province by the Red army made it possible for effect to be given to an already-formulated territorial postulate. The decisions of the three powers, taken in Tehran, Yalta and Potsdam, are also presented and commented upon. The final part of the paper then presents the procedures by which the borderline was delineated, and determined in practice, as this resulted in the ultimate division of East Prussia into Polish and Soviet parts. In particular, attention is paid to the boundary negotiations relevant to that, which took place throughout 1945. Over the longer term, the boundary in fact proved unstable, with individual localities changing their political affiliations from one state to the other. All this of course happened under strong pressure from the Soviet side that effectively left the Polish party helpless. The border that was finally established, largely in the form of a straight line running East-West, in fact persists through to the present day, and now separates Poland from the Kaliningrad District of the Russian Federation.