The coast towns, which were in maritime communication with the ports of the Delta, submitted to the Egyptian yoke more readily than those of the interior. But this submission could not be reckoned on beyond Berytus, on the banks of the Lykos, though occasionally it stretched a little further north as far as Byblos and Arvad; even then it did not extend inland, and the curve marking its limits traverses Coele-Syria from north-west to south-east, terminating at Mount Hermon. Damascus, securely entrenched behind Anti-Lebanon, almost always lay outside this limit. The rulers of Egypt generally succeeded without much difficulty in keeping possession of the countries lying to the south of this line; it demanded merely a slight effort, and this could be furnished for several centuries without encroaching seriously on the resources of the country, or endangering its prosperity. When, however, some province ventured to break away from the control of Egypt, the whole mechanism of the government was put into operation to provide soldiers and the necessary means for an expedition. Each stage of the advance beyond the frontier demanded a greater expenditure of energy, which, with prolonged distances, would naturally become exhausted. The expedition would scarcely have reached the Taurus or the Euphrates, before the force of circumstances would bring about its recall homewards, leaving but a slight bond of vassalage between the recently subdued countries and the conqueror, which would speedily be cast off or give place to relations dictated by interest or courtesy. Thutmosis III. had to submit to this sort of necessary law; a further extension of territory had hardly been gained when his dominion began to shrink within the frontiers that appeared to have been prescribed by nature for an empire like that of Egypt. Kharu and Phoenicia proper paid him their tithes with due regularity; the cities of the Amurru and of Zahi, of Damascus, Qodshu, Hamath, and even of Tunipa, lying on the outskirts of these two subject nations, formed an ill-defined borderland, kept in a state of perpetual disturbance by the secret intrigues or open rebellions of the native princes. The kings of Alasia, Naharaim, and Mitanni preserved their independence in spite of repeated reverses, and they treated with the conqueror on equal terms.* * The difference of tone between the letters of these kings and those of the other princes, as well as the consequences arising from it, has been clearly defined by Delattre.