Complex sentence: While there, he was president of the Chicago Chinese Student Association and his work with the American Anti-Imperialist League led him to join the Chinese Communist Party, which was then partnered with Sun Yat-sen's Nationalist Party.
Split into simpler sentences: While there, he was president of the Chicago Chinese Student Association and worked with the American Anti-Imperialist League. There he joined the Chinese Communist Party, which was then partnered with Sun Yat-sen's Nationalist Party.