Rees was one of the few survivors of his battalion after a 31 October action at Gheluvelt Chateau and assumed command of the unit until January 1915.

Rees was appointed to command the 94th Infantry Brigade on 15 June 1916 and led them in action weeks later on the first day on the Somme.

He led them in a defence against the German spring offensive but the Brigade was over-run during the Third Battle of the Aisne and Rees was captured.

On his return to Britain he commanded the 6th battalion of his regiment and served as a general staff officer before retiring in 1922.

Rees served in the Second Boer War in the Cape and Orange River provinces between July 1901 and May 1902 and was awarded the Queen's South Africa Medal with four clasps for his service.

[1] Rees was serving with the 2nd battalion of the Welsh Regiment by 12 August 1914, when it was sent to France shortly after the outbreak of the First World War.

Rees won the Distinguished Service Order (DSO) during this action for leading the battalion's frontline company to relieve pressure from a German attack on the adjacent South Wales Borderers, whilst under heavy fire.

[2] Rees was appointed to the brevet rank of major on 18 February 1915 in recognition of his service in the field and on 15 March he was seconded to the Royal Military College, Sandhurst.

[2][4] The brigade suffered heavy casualties, Rees saved lives by taking the decision to recall the 1st and 2nd Barnsley Pals (the 13th and 14th battalions of the York and Lancaster Regiment) from a stalled assault.

[4][12] Rees was promoted to the brevet rank of lieutenant-colonel on 1 January 1917, for his service in the field, and afterwards commanded the 13th Reserve Brigade on the Home Front.

[4] Seeing that the German advance could not be halted Rees decided on a withdrawal from a position known as the Plateau de Californie (near Craonne) to PC Terasse (south of Craonnelle).

Upon arriving at his headquarters Rees found that the 1/4th Battalion, Yorkshire Regiment (Green Howards), had been overrun and German troops were already approaching PC Terrasse.

[4] Wilhelm stated that he thought Britain and Germany should be friends and noted his appreciation for the fighting abilities of British troops.