A completely level plain, quite open at the same time, that is, a tract of country which cannot influence war at all, has no existence except in relation to small bodies of troops, and with respect to them only for the duration of some given moment of time. When larger bodies are concerned, and a longer duration of time, accidents of ground mix themselves up with the action of such bodies, and it is hardly possible in the case of a whole army to imagine any particular moment, such as a battle, when the ground would not make its influence felt.
This influence is therefore never in abeyance, but it is certainly stronger or weaker according to the nature of the country.
If we keep in view the great mass of topographical phenomena we find that countries deviate from the idea of perfectly open level plains principally in three ways: first by the form of the ground, that is, hills and valleys; then by woods, marshes, and lakes as natural features; and lastly, by such changes as have been introduced by the hand of man. Through each of these three circumstances there is an increase in the influence of ground on the operations of war. If we trace them up to a certain distance we have mountainous country, a country little cultivated and covered with woods and marshes, and the well cultivated. The tendency in each case is to render war more complicated and connected with art.
The degree of influence which cultivation exercises is greater or less according to the nature of the cultivation; the system pursued in Flanders, Holstein, and some other countries, where the land is intersected in every direction with ditches, dykes, hedges, and walls, interspersed with many single dwellings and small woods has the greatest effect on war.
The conduct of war is therefore of the easiest kind in a level moderately-cultivated country. This however only holds good in quite a general sense, leaving entirely out of consideration the use which the defensive can make of obstacles of ground.