Silla and the Tang destroy Baekje and Goguryeo; Silla unites Korea

In the 640s Silla was hard pressed: in 642 Baekje took more than forty of its fortresses, and the prince Kim Chunchu failed to win help from Goguryeo. In 648 he went to Chang’an, where, according to the Samguk sagi, Emperor Taizong promised him troops; Silla adopted Tang court dress in 649 and the Tang era name in 650, and in 654 Kim Chunchu became king as Muyeol. In 660 a Tang army of 130,000 under Su Dingfang crossed the Yellow Sea while 50,000 picked Silla troops under Kim Yu-sin advanced overland. On 20 August, at Hwangsanbeol, the Baekje general Gyebaek held them off in four battles until the self-sacrifice of two Silla youths, Bangul and Gwanchang, roused their army to victory; the capital Sabi was besieged, and on 29 August King Uija surrendered. The Tang divided Baekje into five commands and carried off the king, 93 members of the royal house and officials and 12,000 commoners; a restoration movement, joined by the prince Buyeo Pung from Japan, collapsed after a Yamato fleet sent to its aid was destroyed at the mouth of the Baek River in 663.

Goguryeo, weakened by decades of war, split after the death of its dictator Yeon Gaesomun in the mid-660s, when his eldest son Namsaeng, driven out by his brothers, went over to the Tang. In 667 the Tang general Li Ji invaded again, and in the autumn of 668 (the ninth lunar month), with Silla troops, he took Pyongyang and captured King Bojang; according to the Old Book of Tang, the kingdom had 170 cities and 697,000 households, which the Tang placed under a Protectorate General to Pacify the East at Pyongyang.

The Tang now treated Silla, too, as a dependency, and Munmu, Muyeol’s son and successor, took up arms against his ally. From 670 he supported Goguryeo loyalists and seized the lands of Baekje; a Tang counter-offensive in 674–675 forced him to apologize, but the war went on, and the Samguk sagi claims a great victory at Maeso in 675 — although historians dispute its scale — while the Tang was increasingly drawn into war with Tibet. In February 676 the Tang moved the protectorate from Pyongyang to Liaodong, and in the eleventh lunar month (December 676 to January 677) the Silla fleet, after an initial defeat, won twenty-two engagements at Gibeolpo at the mouth of the Geum River. Silla was left master of the peninsula up to the Taedong River and Wonsan Bay; in the north the kingdom of Balhae arose in 698, and some historians therefore question whether Silla really unified the Three Kingdoms. For Kim Yu-sin, who died in 673, the Samguk sagi records the words: ‘The Three Han have become one family.’