During World War II, Jasienica (then, Leon Beynar) fought in the Polish Army, and later, the Home Army resistance. Near the end of the war, he was also working with the anti-Soviet resistance, which later led to him taking up a new name, Paweł Jasienica, to hide from the communist government of the People's Republic of Poland. He was associated with the "Tygodnik Powszechny" weekly and several other newspapers and magazines. He is best known for his 1960s books on Polish historyon the Kingdom of Poland under the Piast Dynasty, the Jagiellon Dynasty, and the elected kings of the Polish–Lithuanian Commonwealth. Those books, still popular, played an important role in popularizing Polish history among several generations of readers.