In January 1900, Warren bungled the second attempted relief of Ladysmith, which was a west flanking movement over the Tugela River. At the Battle of Spion Kop, on 23–24 January 1900, he had operational command, and his failures of judgment, delay and indecision despite his superior forces culminated in the disaster. Farwell highlighted Warren's fixation with the army's oxen and his view that Hlangwane Hill was the key to Colenso. Farwell suggested Warren was "perhaps the worst" of the British generals in the Boer War and certainly the most "preposterous". He was described by Redvers Buller in a letter to his wife as "a duffer", responsible for losing him "a great chance".