Ancient & modern
An ancient excerpt and two modern posted without comment, except to say that they are roughly datable in their content to AD 81, 1824 and 1879, respectively.
“Agricola had given protection to one of the chieftains of the Irish who had been driven out by internal conflict, and under the pretence of friendship was holding onto him in case an opportunity presented itself. I have on many occasions heard him say that Ireland could be conquered and held by a single legion and a moderate number of auxiliaries.” (Agricola expulsum seditione domestica unum ex regulis gentis exceperat ac specie amicitiae in occasionem retinebat. saepe ex eo audiui legione una et modicis auxiliis debellari obtinerique Hiberniam posse). Tacitus, Agricola 24.3
“If the British did not get their hands on Afghanistan first, [William Moorcroft] warned, then the Russians almost certainly would. And what better moment than the present, when two rival factions were vying for the Afghan throne? A single British regiment, Moorcroft argued, was all that would be needed to place a suitably compliant candidate on the throne.” Peter Hopkirk, The Great Game, p. 99
“But if [Col. George Pomeroy] Colley [private secretary to the Viceroy of India] was an admirable theorist in the art of war, he did not give me the impression that he would be equally good in practice. He tried to convince me quite seriously that a British regiment 1,000 strong, and armed with Martini-Henry rifles, ought to be able to march through the length and breadth of Afghanistan, when once clear of the Khyber and Kuram Passes.” J. L. Vaughan, My Service in the Indian Army—And After, p. 182
Mountainous countries breed hardy warriors and create logistical difficulties for invaders: Nepal, Switzerland, Scotland and Afghanistan. Only the perversity of the Irish adds them to that illustrious group, as their mountains are paltry. My conclusion: Agricola had better reasons for confidence than did Moorcroft and Colley, based on general precepts. Mind you, the English bagged three of those mountainous countries for a time, so it pays never to underestimate folly and presumption.