I seem to remember that the "agreement" was between Stalin and a different leader -- the infamous Herr Ribbentrop and his master, Herr Hitler. At the time of the invasion of Latvia, Sept 1940, the USSR was bound by nonaggression treaty to the Nazis, with whom they had digested Poland. Mr. Churchill was not exactly party to the USSR's political decision making in this interval.
If you're talking about Yalta, I'd say the west had a different concept of "sphere of influence" than the freedom loving peoples of the USSR. When the allies administered an area, we worked to achieve their independence and self sufficiency. We did not invade them if they displeased us (remember Hungary?) -- as can be seen by France's continued existance as an independent country.
However, I realize that with the pair of brainwash-ees in question, I'm just wasting my breath.
Sigh.