When the last emperor of the Han period had to abdicate in favour of Ts'ao P'ei and the Wei dynasty began, China was in no way a unified realm. Almost immediately, in 221, two other army commanders, who had long been independent, declared themselves emperors. In the south-west of China, in the present province of Szechwan, the Shu Han dynasty was founded in this way, and in the south-east, in the region of the present Nanking, the Wu dynasty. The situation of the southern kingdom of Shu Han (221-263) corresponded more or less to that of the Chungking regime in the Second World War. West of it the high Tibetan mountains towered up; there was very little reason to fear any major attack from that direction. In the north and east the realm was also protected by difficult mountain country. The south lay relatively open, but at that time there were few Chinese living there, but only natives with a relatively low civilization. The kingdom could only be seriously attacked from two corners--through the north-west, where there was a negotiable plateau, between the Ch'in-ling mountains in the north and the Tibetan mountains in the west, a plateau inhabited by fairly highly developed Tibetan tribes; and secondly through the south-east corner, where it would be possible to penetrate up the Yangtze. There was in fact incessant fighting at both these dangerous corners. Economically, Shu Han was not in a bad position. The country had long been part of the Chinese wheat lands, and had a fairly large Chinese peasant population in the well irrigated plain of Ch'engtu. There was also a wealthy merchant class, supplying grain to the surrounding mountain peoples and buying medicaments and other profitable Tibetan products. And there were trade routes from here through the present province of Yünnan to India.