Commanding 20th Brigade
Trefusis was promoted to the temporary rank of brigadier general and took over from Brigadier General Frederick James Heyworth as general officer commanding (GOC) of the 20th Infantry Brigade, part of the 7th Division, on 16 August 1915. This made him at the time the youngest general in the army and the British Empire. Although not formally designated a Guards brigade, his command had contained two Guards battalions, but these just had been withdrawn to join a new Guards Division and replaced by two 'Kitchener's Army' battalions (8th and 9th) of the Devonshire Regiment fresh from England. He also had two Regular battalions (2nd Border Regiment and 2nd Gordon Highlanders) and one Territorial Force battalion (6th Gordon Highlanders) in a very mixed command.
Less than six weeks after his arrival, Trefusis commanded 20th Bde in the Battle of Loos. On the opening day of the British attack (25 September 1915), as the right-hand brigade in 7th Division, 20th Bde's first objective was Breslau Trench, then the successive German lines of Gun Trench and Cité Trench. When Trefusis examined the German wire in front of his brigade on the evening before the attack, he found that it was largely uncut by the artillery bombardment and sent forward parties after dark to cut it by hand. The brigade attacked at 06.00 in a gas cloud – the first British use of chemical warfare – but this was relatively ineffective, and hindered the attacking infantry. However, the artillery had done a good job in front of 2nd Gordons, and within 10 minutes the leading waves of were in the German front trench, although at a heavy cost. In front of 8th Devons, the guns had done less damage and the battalion suffered over 600 casualties. Nevertheless, the two battalions took Breslau Trench and captured some German guns. They advanced to the Lens road, where they were joined by the second wave (6th Gordons and 2nd Border). 2nd Border then took Gun Trench and settled down to defend their gains. Trefusis now sent forward his reserve battalion, 9th Devons, to push on to Cité Trench, but they suffered severely from artillery fire crossing the captured ground, as did the division's reserve brigade.
At 01.00 the following morning, a strong German counter-attack pushed the brigade's advanced posts back to Gun Trench, which had been put into a state of defence. After fierce hand-to-hand fighting, the leading Germans were thrown out of Gun Trench, which became the British front line. Trefusis's brigade had suffered severely on the first day, but on 26 September it maintained its position and its bombers supported a renewed attack by neighbouring British troops on a position known as The Quarries.
When the Guards Division moved up in support on 27 September, 1st Irish Guards found itself behind positioned 20th Brigade, which 'was in no shape for further fighting, but was hanging on in expectation of relief, if possible, from the mixed duties of trying to establish a line and sending out parties to assist in repelling the nearest counter-attack. Fighting continued everywhere, ... and heavy rain added to the general misery' (Kipling). The Irish Guards' historian recorded that Trefusis 'slips, or wades, through rain and mud to lunch with his old Battalion a few hundred yards away ... Then he goes on with the remnants of his shattered Brigade, to take over fresh work on a quieter part of the line, and en route to "get his hair cut".'
