Karl Liebnecht (1871 - 1919) was a German socialist and co-founder of the German Communist Party. Liebnecht was also a pacifist and an opponent of the First World War. With Rosa Luxemburg, he was murdered in the socialist uprising of 1919, while rumours about their assassination had circulated from early in the war. Once such occurrence is commented on in Schreiner’s letters. Schreiner had known his father during the 1880s and had taken an interest in Karl Liebknecht’s political career.
For further information see:
Helmut Trotnow (1984) Karl Liebknecht (1871-1919): Political Biography Connecticut: Archon Books