When the conqueror thinks that "my enemy (now an ally) has to march through an unknown country, which is intersected with mountains, forests, rivers, forts and deserts which is devoid of food-stuffs, people, pastoral grounds, fodder, firewood and water, and which is far away, different from other countries, and not affording suitable grounds for the exercise of his army; and I have to traverse a country of quite the reverse description," then he should make an agreement to carry out a work in a definite locality.