Owing to the strength of this [the Navajo] tribe which numbered then not less than sixty or seventy thousand (60 or 70,000) souls embracing as it did some of those Indians who now call themselves 'Apaches' but who still speak the same language, and who are so alike, and to the fact that they inhabit a country equal to one-third of the whole Territory; that this section was a 'Terra Incognita' and that there is no portion of the American Continent so well adapted by nature for the peculiar style of warfare adopted by the Indians, it is not at all surprising that the many powerful campaigns made against them by the Spanish Government were entirely barren of results as to their subjugation.

Letter to General James Henry Carleton (May 17, 1864) as quoted by Ralph Emerson Twitchell, The Leading Facts of New Mexican History Vol. 3 (1917)