Complex sentence: He joined a revolutionary cell bent on assassinating the Tsar and was selected to construct a bomb; before the attack commenced, the conspirators were arrested and tried, and in May, Sacha was executed by hanging.
Split into simpler sentences: He joined a revolutionary cell bent on assassinating the Tsar and was selected to construct a bomb. However, before the attack the conspirators were arrested and tried, and in May, Sacha was executed by hanging.