After World War II, Loewenstein was rearrested and imprisoned without charge for a period of time by authorities of the reconstituted Czechoslovakia, purportedly due to mistaken identity.
Though a controversial[1] figure owing to his role in the ghetto hierarchy, historian H. G. Adler – another Theresienstadt internee – later credited Loewenstein's discipline and management as bringing "boons to the camp" and stated that his eventual removal from his post led to "misfortune" for the ghetto's inmates.
[1][6] Loewenstein's involvement in the Confessing Church, in combination with his Jewish parentage, led to his arrest in November 1941.
[1] In September 1942, Loewenstein – due to his military experience and German language fluency – was appointed by SS-Hauptsturmführer Siegfried Seidl to organize and lead the Theresienstadt Ghetto's security service, the so-called "Ghetto Guard", making him the second-highest ranking inmate after the Elder of the Jews Jakob Edelstein.
[9] Unlike in other concentration camps, Loewenstein's Ghetto Guard was not used to support SS deportation operations.
Adler who, writing in the foreword to his 1955 work Theresienstadt 1941–1945, recalled that: Even if some may find it disagreeable, it remains a fact that, for a year, Loewenstein – with his great virtues and obvious faults – was a decisive force in shaping the camp's history.
To ignore this would be a gross distortion of the truth; because Loewenstein has been treated quite unfairly in the past, there is all the more reason to emphasize his historical significance here and so try to strip away some of this injustice.
[4] Following the liberation of Theresienstadt by the Soviet Union, Loewenstein was almost immediately arrested and held without charge for two years at various sites, including Pankrác Prison.
[7] He is briefly referenced in Tom Lampert's One Life, an account of eight semi-fictional, composite persons from World War II created from the author's documentary research.