Between 1939 and 1942, Jasper was able to undertake historical research at the city archive. Like other surviving politicians of the Weimar Republic, his anti-Nazi past kept him under watch by the authorities. A failed assassination attempt against Hitler took place on 20 July 1944, which was followed by mass arrests. Jasper was re-arrested on 22 August 1944, under the pretext of what was termed "Aktion Gitter".
