Complex sentence: It was founded and edited until by Karl Kautsky and Emanuel Wurm until their withdrawal from the SPD in 1917; following that, Heinrich Cunow took over as its chief editor.
Split into simpler sentences: It was founded and until 1917 was directed by Karl Kautsky and Emanuel Wurm. After Krautsky and Wurm's withdrawal from the SPD in 1917, Heinrich Cunow took over as chief editor.