Complex sentence: In 1921, he joined the foreign section of the Communist Party of China (Zhongguo Gòngchǎndǎng) by then mainly composed by Jews from Russia and Poland, and attended the first party congress in July 1921 held in the French Concession in Shanghai.
Split into simpler sentences: In 1921, he joined the foreign section of the Communist Party of China (Zhongguo Gòngchǎndǎng) by then mainly composed by Jews from Russia and Poland. He attended the first party congress in July 1921 held in the French Concession in Shanghai.