What has been a serious cause of division among the nations Eastern Europe?

The division of Europe after the Second World War into two blocs, the Western Bloc and the Eastern Bloc, was to some extent a continuation of the division of Europe into two "spheres of influence" that had been agreed between the USSR, the UK and the USA in the so-called "Percentages Agreement" reached at the Yalta Conference in February 1945.

The dividing line was largely along the so-called "Iron Curtain", a term first coined by Winston Churchill in 1946 to refer to the emerging line of communist and Soviet-controlled states in central and eastern Europe. With a handful of exceptions, there was relatively little change to this pattern of East and West European political allegiance until the collapse of the Soviet Union between 1989 and 1991.

The end of the Cold War removed the ideological basis for the division of Europe, and since the 1990s, the European Union has embarked on a policy of enlargement, extending its membership to a number of countries formerly in the Soviet Union and Warsaw Pact.