Third of British Indian army was stationed in NWFP and tribal areas

There are two forms of warfare to be taught in India, viz, open warfare and mountain warfare. Except for those stationed on the frontier the former of course requires the most attention, but mountain warfare should never be entirely neglected in view of the fact that wherever the Army in India fights in the future it is almost certain to be in mountainous country. In addition, about a third of our Army in India is presently stationed on the frontier and practically every unit takes a turn of duty there sooner or later
(“An Infantry Officer”, ‘Collective Training in a Battalion’, JUSII, 60/259, (1930), p.128)
THE ARMY IN INDIA & FRONTIER WARFARE 1914-1939
Military Power: Land Warfare in Theory and Practice edited by Brian Holden Reid