After what he later remembered as "the two most miserable years of my life" at Woolwich Academy, Hankey received his commission as a second lieutenant, joined the Royal Garrison Artillery and was ultimately stationed in Mauritius until serious ill-health led to his return to England on extended sick leave at the end of 1906.

One was his long-standing interest in an eventual career as a Church of England clergyman; another was a recently formed fascination with the challenge of ministering in some way to the manifold needs of the urban poor; and finally, a comfortable legacy at his father's death (1906) gave him the means to make these two objectives practicable.

Accordingly, he spent four months in residence at Rugby House, a mission in one of London's roughest pockets of poverty, and at the same time enrolled in a "crammer" at Charterhouse with the aim of gaining admittance to university and ultimately to ordination in the Church.

Returning to the UK and to Bermondsey in the winter of 1913, Hankey resumed his work with the Mission, looking ahead to a more constructive sojourn in Australia the next summer and throwing himself into the writing of a book on Jesus and the failings of the contemporary church.

He put in for a commission, but hearing that Lord Kitchener had called for one hundred thousand recruits under thirty, Hankey (who was some two months short of that limit) decided that as a "possible parson" he preferred "experience in the ranks", and on 8 August enlisted as a private in the 7th Battalion of the Rifle Brigade.

So given the extreme needs of the new army, Hankey's military experience marked him at once, and within a week he was made a sergeant, then sent to barracks at Aldershot and later billeted upon elderly Mrs. Coppin of Firs Cottage in nearby Elstead.

His few short months there, training recruits and sharing meaningfully in their lives, mediating to some degree between humble men and the rigid authority above them, were among the happiest and most fulfilling days of Hankey's life.

Hankey had found his relationship to his fellow men-in-arms more deeply satisfying even than work among London's poor, and his book subtitled "A study of the greatness of Jesus and the weakness of His Church" gave him needed confidence in his promise as a writer.

In their day, collected in two volumes published in the spring of 1916 by Andrew Melrose and (posthumously) in 1917, the pieces that comprise A Student in Arms were received with what must be called gratitude; today they provide valuable insight into how the 1914–18 war appeared to many who, both as civilians and in the military, actually experienced it whether facing combat or waiting anxiously at home.

His brief first-hand participation in the cataclysmic opening day of the Battle of the Somme and its sickening aftermath mark some of Hankey's final letters with unmistakable signs of shock.