desert Frontier that the greatest captain of modern times, who had himself crossed it at the head of an army, wrote as follows in his Commentaries:—
Generals who have marched from Egypt to Syria or from Syria to Egypt have in all periods of history considered this desert the greater obstacle the larger the number of horses they took with them. The ancient historians declare that when Cambyses wished to penetrate into Egypt he made an alliance with an Arab king, who caused a canal to flow with water in the desert, which evidently means that he covered it with camels bearing water. Alexander sought to please the Jews so that they might help him in the passage of the desert. This obstacle, however, was not so great in ancient times as it is today since towns and villages existed, and the industry of man contended with success against the difficulties. To-day scarcely anything remains between Salihiyeh and Gaza. An army must, therefore, cross the desert successively by forming establishments and magazines at Salihiyeh, Katieh, and El Arish. If this army starts from Syria it must first of all form a large magazine at El Arish, and then carry it forward to Katieh. But these operations are slow, and they give an enemy time to make his preparations for defense. . . . An army defending Egypt can either assemble at El Arish to oppose the investment of this place, or at Katieh to raise the siege of El Arish, or at Salihiyeh: all these alternatives offer advantages. Of all obstacles which may cover the frontiers of empires, a desert like this is incontestably the greatest. Mountains like the Alps take second rank, and rivers the third. If there is so much difficulty in carrying the food of an army that complete success is rarely obtained, this difficulty becomes twenty times greater when it is necessary to carry water, forage, and fuel, three things which are weighty, difficult to carry, and usually found by armies upon the ground they occupy.
Finally, Africa furnishes the crowning illustration of the Great Sahara, which for centuries not only cut off the Mediterranean belt from the rest of Africa, but cut off the entire remainder of Africa from the civilized world. It was not till the voyages of Prince Henry the Navigator that this long period of isolation came to an end.