In October 1914, a few weeks after the start of the
First World War, Dallas became GSO1 (essentially
chief of staff in most modern armies) to Lieutenant General
Sir Henry Rawlinson, who had recently been assigned as
general officer commanding of the newly formed
IV Corps of the
British Expeditionary Force (BEF) on the
Western Front. Promoted in January 1915 to temporary brigadier general, and made a brigadier general, general staff of the corps, now the senior officer on the staff, he was appointed a
Companion of the Order of the Bath the following month. In his new role he helped in the planning for the major engagements in which IV Corps found itself involved in that year, most notably the
Battle of Neuve Chapelle in March and the
Battle of Aubers Ridge in May. Rawlinson made staff changes in August and Dallas, promoted once again to temporary brigadier general (having since reverted to his permanent rank of colonel) in September, became commander of the
11th (Northern) Division's
32nd Infantry Brigade, which he led in the final stages of the
Gallipoli campaign. and became GOC of the
53rd (Welsh) Division, a
Territorial Force formation, at the time serving in
Egypt after having been recently evacuated from Gallipoli. After having had his major general's rank made substantive in January 1917, he was appointed to the
Order of the White Eagle, 2nd Class by the
King of Serbia in February. He saw action with his division at the
First and
Second Battles of Gaza, in which his division suffered significant losses, in March and April. Aa a result, he handed over command of his division in April. ==Final years==