He was confirmed as general staff officer, grade 1 (GSO1), of the 2nd London Division on 5 August, the day after the
British entry into the
First World War. As the division's senior staff officer, he spent the first months of the war in Britain and helped in the initial organisation and training of the division before it was deployed to the
Western Front to join the
British Expeditionary Force (BEF) in March 1915, and was soon after redesignated as the 47th (2nd London) Division. He continued in this role during the division's initial engagements at the battles of
Aubers and
Festubert. After being promoted to the temporary rank of brigadier general in June 1915, he took command of the
141st (5th London) Infantry Brigade, part of the 47th Division, taking over from its previous commander, Brigadier General
George Colborne Nugent, who had been killed. , at The Brasserie,
Fonquevillers, France, March 1917. Showing Strombos horn and shell case gas alarms. On 23 May 1916, "when the brigade headquarters was at Cabaret Rouge", southwest of
Lens, he "was wounded at 11.30 pm. It also reported that he was wounded at 7.47 am. on the following day". However, as command of the brigade had by now passed to Lieutenant Colonel Tredennick "at 12.30 am on the 24th" he was most likely wounded on the 23rd. He remained in command of the brigade for over a year until being promoted to the temporary rank of major general and made
general officer commanding (GOC) of the
46th (North Midland) Division, another TF formation, in July 1916. His major general's rank became substantive in June 1918 "for distinguished service in connection with Military Operations in France and Flanders". Towards the end of the war Thwaites became
director of military intelligence at the
War Office in
London from September 1918, taking over from Lieutenant General
Sir George Macdonogh. ==Post-war and final years==