A highly decorated officer, Sadleir-Jackson was wounded several times during his military service and later went on to become Inspector of Levies in British territories in the Middle East.

[4] During the war he was posted as a staff officer and saw extensive action, including at the battles of Belmont, Modder River and Magersfontein and Paardeberg, culminating in the relief of Kimberley.

In heavy fighting around the town of Albert, Sadleir-Jackson's men took their objectives led from the front by their Brigadier, who was wounded in the knee by machine gun fire near Bellevue Farm.

Although Sadleir-Jackson attempted to return to British lines he collapsed and was brought in by his men, the wound proving serious enough to keep him in England until after the Armistice.

[4] For his conduct during the attack he was presented with a bar to his Distinguished Service Order in September 1918:[33] For conspicuous gallantry and devotion to duty.

Throughout recent operations he proved himself a bold leader of men, and under all conditions full of energy and fine fighting spirit.

He, personally organised and led most successful counter-attacks, in one of which he recaptured a village and took 150 prisoners and eleven machine guns.In the autumn of 1918, Sadleir-Jackson was rewarded for his war service with the presentation of the Legion d'honneur[34] and the Croix de Guerre with Star.

[4] In the event, Sadleir-Jackson found himself operating independently and frequently in overall charge of the North Russia Campaign which suffered from poor morale and supply and a lack of political direction.

In a matter of months the situation was so critical that by September 1919 the entire force was evacuated, abandoning the region to the Bolshevik armies.

[5] Nevertheless, in the months of fighting which became particularly desperate in August 1919, Sadleir-Jackson's men performed well and fought hard, two Australian volunteers in the 45th Battalion of the Royal Fusiliers even winning the Victoria Cross: Corporal Arthur Percy Sullivan and Sergeant Samuel George Pearse.