Complex sentence: The 1918--1933 Weimar period, which is also called German Reich, was ignored and denounced by the Nazis as a historical aberration, contemptuously referring to it as "the System".
Split into simpler sentences: The 1918--1933 republic, which was also called German Reich, was ignored and denounced by the Nazis as a historical aberration. The Nazis also contemptuously referred to it as "the System".