William Alexander was a leading activist within the Communist Party of Great Britain (CPGB). He was best known for commanding the British Battalion of the International Brigades during the Spanish Civil War. During World War II he served in the British Army, with deployments in Northern Africa, Italy, and Germany. He was promoted to the rank of captain in the Reconnaissance Corps. After the war he became an author, the vice-chairman of the International Brigade Association, and the president of London's Marx Memorial Library. He spent the remainder of his life promoting Marxism–Leninism, and was a member of the CPGB until the Party was dissolved in 1991.