He joined the British Army, receiving his commission as a probationary second lieutenant on 2 May 1916 in the 5th (Reserve) Battalion of the Grenadier Guards,[3] the same regiment with which an ancestor of Adair's had been serving when he was killed at the battle of Waterloo.

[4] From January 1917 onwards he served in the trenches of the Western Front in France and Belgium as part of the 2nd Company, 3rd Battalion, Grenadier Guards, with the rank of lieutenant.

He led his company correctly into position and then made several personal reconnaissances under heavy machine-gun and rifle fire, and cleared up the situation.

[3] After the armistice of 11 November 1918 Adair's battalion returned to London, where on 29 June 1920 he received his permanent lieutenant's commission, with seniority backdated to 2 August 1918.

[6] He was promoted to major on 22 May 1932, and returned to the 3rd Battalion to serve as second-in-command until April 1940, seven months after the Second World War broke out.

[3][7] The battalion, forming part of the 1st Guards Brigade of the 1st Infantry Division (the former commanded by Brigadier Merton Beckwith-Smith and the latter by Major-General Harold Alexander, both, like Adair, Guardsmen), soon found themselves in the thick of the fighting during the battles of Belgium and France, and held the perimeter against German attacks during the Dunkirk evacuation.

[8] The 1st Guards Brigade briefly served under the control of the 5th Infantry Division, commanded by Major-General Harold Franklyn.

[7] It was when Adair's battalion launched a counterattack which helped restore the British line which impressed Franklyn, who later wrote: I give this example of the highest form of discipline.

After marching well over twenty miles on a very hot day they arrived at my Headquarters at 7.30 p.m. An hour later they were put into a vital counter-attack in the half light, over unknown ground.

[11] Following the Allied break-out from Normandy they advanced across Northern France and into Belgium as part of Lieutenant General Brian Horrocks' XXX Corps.

However the burden of its upkeep and maintenance, combined with heavy death duties meant that he was obliged to sell the property in 1950.