In all ages the feeding of his troops in an enemy's country has been one of the gravest problems a military commander has had to solve; and though, in some instances, vast armies have succeeded in drawing sufficient support from the land they have invaded, there have been others in which an army intending to "live upon the country" has failed to get the food it needed, and has had its numbers depleted to the extent of thousands as the result of sheer starvation. This was the experience of Darius, King of Persia, who, in 513 B.C., crossed the Bosporus, on a bridge of boats, with an army of 700,000, followed the retreating Scythians, and lost 80,000 of his men in wild steppes where no means existed for feeding them. When, also, Alexander the Great was withdrawing from India, in 325 B.C., two-thirds of his force died on the desert plains of Beluchistan from thirst or hunger. Lack of the supplies from which he found himself entirely cut off was, again, a main cause of the disaster that overtook Napoleon in his Russian campaign. Even fertile or comparatively fertile lands, satisfying the needs of their inhabitants in time of peace, may fail to afford provisions for an invading army, either because of the great number of the latter or because the retreating population have destroyed the food supplies they could not take with them into the interior whether for their own sustenance or with a view to starving the invaders. Should the invading army succeed in "living on the country," the effect of leaving the troops to their own resources, in the way of collecting food, may still be not only subversive of discipline but of strategic disadvantage through their being scattered on marauding expeditions at a time when, possibly, it would be preferable to keep them concentrated. General Friron, chief of the staff of Marshal Masséna, wrote concerning Napoleon's campaign in Portugal:-- The day the soldier became convinced that, for the future, he would have to depend on himself, discipline disappeared from the ranks of the army. The officer became powerless in the presence of want; he was no longer disposed to reprimand the soldier who brought him the nourishment essential to his existence, and who shared with him, in brotherly goodwill, a prey which may have cost him incalculable dangers and fatigues.