sentences:  Rothstein occupied the left-wing of the party as a prominent theorist and forward thinker, and in 1900 he was elected to its executive.Within the SDF's successor, the British Socialist Party (BSP), he was a leader of the opposition to Hyndman's support for the war.After Hyndman and his supporters left the BSP, Rothstein became a leading figure in the formation of the Communist Party of Great Britain.