In 1918 Poland decided to unilaterally invade modern Ukraine and Belarus because of nationalist expansionist beliefs, you can read about it on the wikipedia article or the Polish-Ukrainian war.

In 1939 the Soviet Union recovered those territories and gave them back to the Belarusian and Ukrainian SSRs, which Poles are still crying about. If you care about seeing maps representing this I can give you sources no problem since I’m arguing from good faith.

After 1945, Poland was forced by the USSR to give reparations in order to compensate for the lack of mutual defense agreement against Nazis before 1939, which was a mistaken policy that led to a lot of insatisfaction in Poland. After the mid-1950s, the Soviet Union removed these reparations and instead started subsidising the Polish socialist nation with cheap resources and industrialising it.

What part of 1969 is relevant to you?