H. W. Halleck,
Elements of Military Art and Science
“ Many circumstances influence this selection, such as mountains, rivers, roads, forests, cities, fortifications, military dépôts, means of subsistence, &c. If the frontier of a state contain strong natural or artificial barriers, it may serve not only as a good base for offensive operations, but also as an excellent line of defence against invasion. A single frontier line may, however, be penetrated by the enemy, and in that case a second or third base further in the interior becomes indispensable for a good defence. ”
