Goguryeo–Sui War

However, in Goguryeo, one of the Three Kingdoms of Korea, king Pyeongwon and his successor, Yeongyang, insisted on maintaining an equal relationship with the Sui dynasty.

After receiving the message, Yeongyang launched a joint pre-emptive invasion with the Malgal against the Chinese along the border in present-day Hebei province in 597.

Emperor Wen ordered his fifth and youngest son, Yang Liang (assisted by the co-prime minister Gao Jiong), and Admiral Zhou Luohou (周羅睺), to invade and conquer Goguryeo with an army and navy totaling 300,000, mostly composed of horse archers.

Coming to the conclusion that the army could not achieve the objective on its own, Yang decided to combine with Zhou's naval fleet and proceed.

Zhou's navy also came across their own challenges, contending with rough seas, losing many ships despite staying close to the coastline.

Emperor Yang of Sui, when he won the throne in 604, built the Grand Canal connecting the north and south of China.

Yeongyang gave up the buffer zone that he had acquired after the war of 598, as the area was not suitable to face such a vast army.

The bridges however, were too short to reach the opposite bank in another fortunate turn of events for the defenders, and Goguryeo's attack successfully held off the first assault.

After the extension of the bridges however, the Sui forces succeeded in crossing the river, and the army surrounded the fortresses of Goguryeo.

Emperor Yang ordered his generals before the campaigns not to make individual decisions regarding the movement of troops, but to report to him to receive instruction.

During the strategically important campaign against Yodong fortress, the general commanding the siege had to send a messenger to the emperor for the approval of the surrender.

By June, five months after the start of the campaign, not a single castle, fortress, or citadel has fallen to Sui.

The Sui naval commander, Lai Huni, considering it to be a chance to add to his merit, led a host of more than 40,000 to lay assault against Pyeongyang.

When the Sui soldiers reached one of the outer castles, however, they found the wall empty and the gate open.

Generals Wu and Yuwen had their own secret order from Emperor Yang to seize Yeongyang or Eulji Mundeok if they should fall into their hands.

However, the generals' advisers, notably the assistant director of the Right of the Department of State Affairs, Liu Shilong (劉士龍), convinced them it would be unwise to arrest negotiators from the opposing camp.

Yu Zhongwen later recognized that his advancing troops were suffering from exhaustion, and that fortifications of Pyeongyang made for an impossible campaign.

Eulji Mundeok sent him for the occasion a poem: Your divine plans have plumbed the heavens; Your subtle reckoning has spanned the earth.