During World War II, Kłoczowski was a soldier of the Home Army (Armia Krajowa) and participated in the Warsaw Uprising, where he was seriously injured and lost his right hand.
Professor Kłoczowski became a target of a slanderous controversy,[9] resulting from an article published in Najwyższy Czas!
Michalkiewicz alleged that Kłoczowski could be the same person as the SB security agent pseudonym "Historyk" (Historian) recruited in 1961, and listed in the secret communist archives held by the Institute of National Remembrance.
His article resulted in a widely publicized official protest printed by Rzeczpospolita on November 13/14, 2004, and signed by 35 academics and prominent politicians such as Tadeusz Mazowiecki, Władysław Bartoszewski and film director Andrzej Wajda.
The letter of protest explained that the slanderous memo (known as "Fałszywka" in Polish) was invented by the security service itself.