Russia's eastward movement through Asia has been strikingly illustrative of her strong craving for free access to southern seas and of the impediments she had experienced in gratifying that wish. An irresistible impulse had driven her oceanward. Checked again and again in her attempts to reach the Mediterranean, she set out on a five-thousand-miles march of conquest right across the vast Asiatic continent towards the Pacific. Eastward of Lake Baikal she found her line of least resistance along the Amur, and when, owing to the restless perseverance of Muravief, she reached the mouth of that great river, the acquisition of Nikolayevsk for a naval basis was her immediate reward. But Nikolayevsk could not possibly satisfy her. Situated in an inhospitable region far away from all the main routes of the world's commerce, it offered itself only as a stepping-stone to further acquisitions. To push southward from this new port became an immediate object to Russia. There lay an obstacle in the way, however; the long strip of sea-coast from the mouth of the Amur to the Korean frontier--an area then called the Usuri region because the Usuri forms its western boundary--belonged to China, and she, having conceded much to Russia in the matter of the Amur, showed no disposition to make further concessions in the matter of the Usuri. In the presence of menaces, however, she agreed that the region should be regarded as common property pending a convenient opportunity for clear delimitation. That opportunity came very soon. Seizing the moment (1860) when China had been beaten to her knees by England and France, Russia secured final cession of the Usuri region, which now became the maritime province of Siberia. Then Russia shifted her naval base on the Pacific from Nikolayevsk to Vladivostok. She gained ten degrees in a southerly direction.