In the immediate aftermath of the Second World War, things go poorly for Stalin as he tries to assert Soviet power in the countries of Eastern Europe. Specifically, Tito is defiant and, while pro-Moscow regimes take power in Bulgaria and Romania, armed resistance is widespread and very difficult to stamp out.
Over the next decade, a particularly irrendentist regime in Athens also causes trouble in the south. The glorious, bloody, and supremely wasteful moment of the Greek dictatorship was not, however, a war against Turkey to reclaim ancestral Greek territory, but instead a war against Bulgaria over the subject of war reparations. Greece lost the war, thanks to the involvement, but the effect in Bulgaria was widespread nationalist discontent. Within the Communist Party itself, Titoism was being voiced more openly.
A few years later, when a revolution kicked off in Hungary, an insurrection was sparked in the Balkans - one which received fairly open Yugoslav support. Unfortunately, Moscow did not have the resources to deal with it on time; things were escalating around the world, with major crises and potential flashpoints in Iran, the Taiwan Strait, the Suez Crisis, and of course Hungary. Everywhere, there were allies that were either not behaving themselves or failing to manage socialism.
There was a war in the Balkans, with Yugoslavia, a newly Marxist-Leninist Greece, and nationalist Bulgarian forces managing to establish a regional bloc under significant influence from Sarajevo. The loyalty of Hungary and Romania (also restive) was finally assured, but the situation wasn't stable. It set things up for a second war which some call the "Third World War in Europe", but which most anglophone academics now call the Superpower-Ending War (based on the most commonly used term in Germany).
Starting in 1969, uprisings swept across Europe on both sides of the Iron Curtain, principally in the Mitteleuropa from France to Poland. At the very same time, relations with the People's Republic of China had deteriorated to the point of war on the Amur River and in Xinjiang. China used nuclear weapons, but they didn't have enough to trigger total annihilation. Moscow's use of nuclear weapons was very limited, considering its arsenal, but very effective. It was not willing to use nuclear weapons in Greece, however, when the Yugoslavs and the Greeks struck, with Tito proclaiming common cause with Mao Zedong and saying that he was the defender of European revolutions.
A naval blockade of the Bosphorus was attempted by Greece, but NATO was slow to act in Turkey's defense because they saw the effects on Moscow. A pro-Soviet regime in Iran was overthrown in a coup. Trade sanctions were imposed. All of this at a time when the Soviet Union was tied up on two enormous fronts. Ultimately, the Soviet Union and Tito broked a ceasefire, which led to both the retreat of all military forces in Eastern Europe to their country of origin. (Greece, fighting a separate war from a legal point of view, was excluded.) The idea in the 1970 peace treaty, signed a few months later, was that only currently non-NATO states that consented to the forces of a non-NATO state on their territory would have them stationed there. By this time, every regime in Eastern Europe, except for East Germany and Poland, had been overthrown.
For all the withdrawal of troops, Eastern Europe remained relatively friendly to the Soviet Union and relatively hostile to the capitalist West. No regime that came to power was explicitly pro-capitalist. In 1978, though, that changed. In Czechoslovakia, a "third way" advocate came to power that resulted in an enormous marketization of the country's economy. He was also very friendly to the West, though not necessarily to the United States. This concerned the regime in Poland, which had remained close to Moscow but nonetheless asked for a withdrawal of Soviet forces in 1970. Now, they wanted them to return.
This moved proved unpopular - not only among the population, but also in the armed forces. What followed was a civil war.
BOOM! South Poland is recognized by certain West European countries, pro-American dictatorships in Africa and Latin America, and obviously Czechoslovakia and the United States (which was covertly involved), whereas North Poland is the more widely recognized regime around the world. It is, indeed, home to Soviet troops who, for the record, never actually got involved in the fighting. South Poland is the smaller of the two, with the border consisting of exactly where troops were positioned when the ceasefire was called in 1982. Both are nasty dictatorships, but the North is probably a bit more hellish.