"From Stettin in the Baltic to Trieste in the Adriatic, an iron curtain has descended across the Continent. Behind that line lie all the capitals of the ancient states of central and eastern Europe. Warsaw, Berlin, Prague, Vienna, Budapest, Belgrade, Bucharest and Sofia, all these famous cities and the populations around them lie in what I might call the Soviet sphere, and all are subject, in one form or another, not only to Soviet influence, but to very high, and in some cases increasing measure of control from Moscow." - Winston Churchill, British Prime Minister
A historian could best use this quote to study the beginning of which of the following?