In the latter conflict he was the head of the British Special Operations Executive (SOE) liaison mission to the Chetnik forces of Draža Mihailović in Yugoslavia from July 1943 to early 1944.

After attending the Royal Military College, Sandhurst,[3] he was commissioned into the East Surrey Regiment in April 1915 during the First World War.

The citation for his MC, appearing in The London Gazette in January 1920, reads as follows: For gallantry and devotion to duty during operations of 10th-11th August, 1919.

At Sludka, on 10th August, he formed a rear-guard defence with his two guns against a party of sailors which had landed and cut off the main body from reserves.

[6]He was present in Mesopotamia 1920-21 and on the Indian North West Frontier 1930-31,[3] serving much of his inter-war service with the 1st Battalion, East Surrey Regiment in Egypt, Sudan, Hong Kong and India.

[5] In July 1943 Armstrong was appointed Brigadier in command of the SOE sponsored Military Mission to the Chetniks, a Serbian nationalist and royalist force led by General Draža Mihailović, and one of the principal resistance movements in Axis occupied Yugoslavia.

[13] Armstrong and Seitz took part in the Chetnik action against Axis controlled units protecting the railway bridge over Drina in Višegrad.

[12] In spite of protests, including from Kenneth Pickthorn and officials of the Yugoslav government in exile in London, the BBC did not make any corrections.

[20] Archival evidence, published in 1980 for the first time, confirms that some actions against Axis carried by Mihailović and his Chetniks with Armstrong were mistakenly credited to Tito and his Communist forces.

[25] Armstrong inherited his predecessor’s (Colonel Bailey) task to convince Mihailovic to increase his anti-Axis activities without providing him with adequate supplies.

[30] Leaving Yugoslavia, Armstrong returned to England and, in June 1944, took command of the British Airborne Liaison Unit to the Polish Parachute Brigade.