The casual reader should receive the message that the expulsions were agreed to at the Yalta and Potsdam conferences.  They should also be aware that Stalin outmaneuvered the gullible Western Allies at both those conferences.  Stalin didn't care about the Poles (he killed 22,000 Poles, mostly Polish military officers, at Katyn in 1940); Stalin was more concerned about gaining back lands lost in the early 1920s, namely lands within the 1937 Polish border east of the Curzon Line.  That land area had a population which included 40% ethnic Poles.  Stalin needed somewhere to dump those Poles, so he proposed that Germans be moved west & the Poles be moved west.  Roosevelt was dying in 1945 - he was easily outmaneuvered by Stalin.  By the Potsdam Conference in the summer of 1945 Churchill had lost power and had to leave the conference early, and Truman was tied down by Roosevelt's commitments, so Stalin got his way again.   The casual reader should also receive the message that the "Wild" expulsions by both Russian and Polish authorities were begun east of the Oder-Neisse Line and long before the end of July 1945 Potsdam Conference which ONLY authorized the ethnic cleansing of Germans from within the 1937 borders of Poland, and from Czechoslovakia and Hungary.  The Potsdam Agreement separated all German territory east of the Oder-Neisse Line from "Occupation Germany" (all of that German territory east of the Oder-Neisse Line had been part of the Soviet Occupation Zone of Germany prior to the Potsdam Conference).  Most of that German territory was placed under temporary Polish Administration pending the final Peace Treaty for World War 2.  The Potsdam Agreement also stipulated that Northern East Prussia to be  placed under temporary Soviet Administration pending the final Peace Treaty for World War 2.  Note: there is a distinction between "Occupation" and "temporary Adminsitration" as delineated by the Potsdam Agreement.