Complex sentence: The exact motives of the small Hamburg group led by Hugo Urbahns and Hans Kippenberger, who planned the uprising, remain unknown; they may have been trying to force the issue with the KPD leadership.
Split into simpler sentences: The exact motives of the small Hamburg group led by Hugo Urbahns and Hans Kippenberger, who planned the uprising, remain unknown. They may have been trying to force the issue with the KPD leadership.