EUROPE agreed to massive annexations of territory and population transfers
The USSR and the other countries carrying out population transfers were Communist-"Leftist" states. But Roosevelt and Churchill had basically agreed to this policy at the Yalta conference in February 1945.
Curiously, when Israel might be the beneficiary of resettling the Arab refugees from 1948, then the Western powers all of a sudden sprout humanitarian principles. The Arab 1948 refugees must be kept in camps funded with Western money (mostly USA $$) until a political settlement with Israel, all the while these refugees and their descendants being used as a moralistic political reproach against Israel.
Nevertheless, the territorial changes in Europe after WW 2 were accepted by the European states at the Helsinki Conference of 1975. All European states but Albania were represented at the conference.
The Helsinki Agreement, Final Act, 1975, also called the Declaration of Principles Guiding Relations between Participating States, declares:
Clause 4 - "The participating states will respect the territorial integrity of each of the participating states. Accordingly, they will refrain from any action. . . against the territorial integrity, political independence, or the unity of any participating state."
This means that 35 European states committed themselves to respecting the border and territorial changes made in Europe at the end of the War which had displaced or "transferred" millions of Europeans. But in the Middle East they have other principles.