Communism came to Poland, but was never invited. Stalin's pseudo-Polish 'Union of Polish Patriots', headed by Boleslaw Bierut, grabbed power as the retreating Nazis relinquished it. Teaming up with the domestic product, the 'Polish Worker's Party' was formed and headed by Wladyslaw Gomulka. To signal a new era, the communist rulers removed the crown from the Polish eagle. In 1947, a faked election let the world know that Poland 'chose' communism, and in 1948 the ruling 'Polish United Worker's Party' (PZPR) was established.
Regardless of name, the game was the same. The Soviet Union was determined to maintain its expanded sphere of influence as the Cold War commenced, and it did so by hook and crook for the next 40 years.
Occupied again: The People's Republic
Unlike the Soviet Union proper, Poland did experience some internal independence. For instance, deposed leaders were not assassinated, purges stopped short of outright genocide, and suppression only went so deep. Most importantly, the Church survived and even flourished as a counterpoint to Soviet repression. Things weren't all bad: Poland did manage to rebuild its war-devastated iron, steel, shipping, and mining industries. But it never regained a decent standard of living and it was that failure, primarily in the form of sky-rocketing food prices, which eventually toppled Soviet rule.
The first sign of discontent surfaced in 1956 when Khrushchev opened the door himself by admitting Stalin's crimes in February of that year. By June, strikes broke out in Poznan, and in October a reform-promising Gomulka was elected without the stamp of Moscow approval. This unheard of defiance elicited a visit from Khrushchev coupled with several armies massing at the Polish border, but Gomulka effectively deflected the threat. The openness and reforms which followed lasted about as long as any decent cynic would expect, and quickly enough, everything went back to normal.
After another decade of this, high food prices again sparked unrest in Gdansk in 1970 but this time around the solution proved more dangerous.
Earlier that year, the chancellor of West Germany, Willy Brandt, visited Poland and opened the east to the west for the first time since the war. Grabbing the opportunity, Party Secretary Gierek began borrowing money, and he borrowed Poland into debt, leaving it far worse off than before. When the food prices announced several years later proved far higher than those which sparked the borrowing in the first place, Gierek was ready.
By suppressing all resistance, the government ensured the birth of the eventually fatal union between the workers, the intellectuals, and the Church. Historically, once the students and the workers get together with the clergy, serious counter-trouble starts.
And did, in the early 1980's. Unrelated, yet so connected, was the election of the Polish-born Pope John Paul II in 1979. Spiritually supported, Poles became bolder. Another price rise sparked yet another strike in Gdansk, and continuing unrest paved the way for some cooperation. The Gdansk shipbuilders wrote out their demands - the 21 Points - and the government agreed to them in August of 1980. With that, the Solidarity Trade Union was born.
Unfortunately, Poland's neighbors didn't take kindly to this development, started complaining to the Kremlin, and the countermeasures began. They ended with martial law in December of 1981: Solidarity was banned, its leaders jailed, and life went back to normal. But not for good: the underlying discontent with life under Soviet hegemony resurfaced again and again, until strikes in 1988 forced the government to negotiate. The Round Table Talks followed in 1989 and soon thereafter the first noncommunist government in Central Europe since WWII was installed.