It was evident, therefore, that Japan's first object was to shut off Port Arthur from the sea, and her next to cut the railway communication to the North. This done, the Russian fortress, however impregnable to assault, must ultimately fall to investment. From Port Arthur, which, as a glance at the map will show, lies at the very tip of Liao-tung Peninsula, the railway runs due north for six hundred miles through Niuchwang and Mukden to Harbin, where it joins the branch line to Vladivostock. Though Russia has for several years been in occupation of this territory, her hold upon it is by no means secure. The population is distinctly unfriendly, and for the mere defence of the line thousands of troops are necessary. Indeed, it was this necessity that Russia urged as an excuse for her military occupation of Manchuria. [Sidenote: Korea as Base] Within the triangle of which Harbin is the apex, of which the lines to Port Arthur and Vladivostock are sides, and of which the course of the Yalu River is the base, the sphere of immediate military operations practically had to be confined, as the ice-bound condition of the coast to the west of Port Arthur made a landing in force there impossible till the spring. The necessity of maintaining communications tied the Russian forces very largely to the railway lines. But for either belligerent the helpless kingdom of Korea, which lies south of a line drawn between Port Arthur and Vladivostock, for aggressive operations, afforded the most convenient line of advance. Through Korea Russia could menace Japan, and through Korea Japan could most easily march against Port Arthur. Naturally, therefore, Russia's first care was to mass her available troops on the line of the Yalu, and concentrate reinforcements at Harbin ready to be moved to whatever point might prove the objective of the Japanese attack. [Sidenote: Command of the Sea] But the command of the sea was the essential condition to attack by land by either combatant. With the Russian fleet masked or destroyed, Japan could choose as a landing-place for her armies any of the numerous ports on the western coast of Korea, and so approach in force the Yalu River, which divides Korea from Manchuria and the Liao-tung Peninsula. With imperfect command of the sea, Japan would have a second resource. She could land her troops at Masampo, separated only by a hundred miles of sea from her own ports, or she could, at a push, land her forces on the east coast of Korea, at Yuen San or Gensan. But the former plan of operations would have entailed a long overland march before the objective was reached, and the latter the maintenance of communications over difficult and mountainous country. Evidently, then, immeasurable importance attached to the result of the first naval engagements, and to their influence in giving the command of the sea to the one or the other of the two belligerent Powers. [Sidenote: The First Blow]