In terms of Yugoslavia, it wasn't so much Stalin and Hitler that kept the lid on Yugoslavia, it was Tito. And there wasn't a decline in ethnic diversity as the wars in Yugoslavia attested to. The divides are not not just ethnic but religious and political and the term Balkanization, which refers to the geographic division of a region creating political instability, was derrived from this region. As for other areas, in central Europe, Hitler and Stalin actually exsaserbated the ethnic divisions between Czechs and Slovaks and Hungarians but the manifestation of those divisions was delayed until the break up of Commecon.
As regards Yugoslavia, this is precisely what Judt is saying - despite all appearances to the contrary, it remained a powder keg of potential ethnic conflict, because the various ethnic peoples had not been forcibly relocated, nor otherwise migrated, to their own "countries" of origin, during or immediately after World War II. As Judt points out, this was not the case in most of Europe.
Judt writes, "Czechoslovakia, whose population before Munich was 22 percent German, 5 percent Hungarian, 3 percent Carpathian Ukrainians and 1.5 percent Jewish, was now [after WWII] almost exclusively Czech and Slovak: of the 55,000 Czechoslovak Jews who survived the war, all but 16,000 would leave by 1950" (Judt,
Postwar, p. 28). In the aftermath of the war Czechoslovakia had forcibly deported almost 3 million ethnic Germans from its borders; over 250,000 died in being transferred to Germany. The same pattern was repeated with little variation in Poland and Hungary.
"Some Western observers were shocked at the treatment of the German communities [in Central and Eastern Europe]. Anne O'Hare McCormick, a
New York Times correspondent, recorded her impressions on October 23rd, 1946: "The scale of this resettlement, and the conditions in which it takes place, is without precedent in history [sic, sic, sic, as Judt points out repeatedly, both Stalin and Hitler had certainly set plenty of precedents for the forcible relocation of entire populations - E.] No one seeing its horrors first hand can doubt that it is a crime against humanity for which history will exact a terrible retribution" (Judt. p. 26).
The subsequent lessening of ethnic tension in Czechoslovakia is testified to by the fact that in the early 1990s, with the breakup of the former Soviet bloc, this country divided peacefully, without bloodshed, into the newly independent Czech Republic and the republic of Slovakia. This political accomplishment was made easier because the ethnic populations in question were roughly grouped together and roughly located within specific territorial boundaries. This was in total contrast to Yugoslavia, where different ethnic and religious populations were spread out and overlapped virtually everywhere. As a result, as we all know, with the collapse of communism Yugoslavia dissolved into civil war and "ethnic cleansing."