It wasn't. Mentions of the Polish-Soviet conflict were excised from history books and teachers attempting to mention it were persecuted in Bierut (Stalin's) times.

For instance, the first Polish edition of Norman Davies' "God's Playground" was released in an interesting period - the second tome was released after communist censorship was abolished. The first one has all references to Soviet-Polish conflict carefully excised, which was later demonstrated in the second tome.