Complex sentence: While he used his office to mitigate some of the worst effects of the Nazi occupation, he negotiated with the most important exponents of the Nazi regime in Czechia, and his role was therefore assessed negatively by the Czech public.
Split into simpler sentences: While he used his office to mitigate some of the worst effects of the Nazi occupation, he negotiated with Nazism's representatives in Czechia. His wartime role was therefore assessed negatively by the Czech public.