Complex sentence: In London, where the family was reduced to poverty, Marx continued writing and formulating his theories about the nature of society and how he believed it could be improved, and also campaigned for socialism--he became a significant figure in the International Workingmen's Association.
Split into simpler sentences: In London, where the family was reduced to poverty, Marx continued writing and formulating his theories about the nature of society and how he believed it could be improved. He also campaigned for socialism and became a significant figure in the International Workingmen's Association.