Interesting question as to why the USSR didn’t to it again. Maybe they were facing internal problems.
The younger generation had different ideals than the old farts in control. Perhaps believed in freedom?
I think they knew the 50 years was nearly up, and it wouldn’t be worth the political cost of military action if they only had a few years left before the “secret treaty” ended.
“However, there was also a secret protocol to the pact, revealed only on Germany’s defeat in 1945, according to which the states of Northern and Eastern Europe were divided into German and Soviet spheres of influence. In the North, Finland, Estonia and Latvia were apportioned to the Soviet sphere. Poland was to be partitioned in the event of its “political rearrangement”—the areas east of the rivers Narev, Vistula and San going to the Soviet Union while the Germans would occupy the west. Lithuania, adjacent to East-Prussia, would be in the German sphere of influence. In the South, the Soviet Union’s interest and German lack of interest in Bessarabia, a part of Romania, were acknowledged. The German diplomat Hans von Herwarth informed his U.S. colleague Charles Bohlen on the secret protocol on August 24, but the information stopped at the desk of President Franklin Roosevelt.
Concerns over the possible existence of a secret appendix were first expressed in the intelligence organizations of the Baltic states scant days after the pact was signed, and the speculations grew stronger when Soviet negotiators referred to its content during negotiations for military bases in those countries. The German original was presumably destroyed in the bombings, but its microfilmed copy was included in the documents archive of the German Foreign Office. Karl von Loesch, a civil servant in Foreign Office, gave this copy to British Lt. Col. R.C. Thomson in May 1945. The Soviet Union denied the existence of the secret protocols until 1988, when politburo member Alexander Nikolaevich Yakovlev admitted the existence of the protocols, although the document itself was declassified only after the dissolution of the Soviet Union in 1992. In December 1989, the first democratically elected Congress of Soviets “passed the declaration admitting the existence of the secret protocols, condemning and denouncing them”
http://en.wikipedia.org/wiki/Molotov-Ribbentrop_Pact#Alternative_terms