A series of campaigns launched by the Sui Dynasty of China against the Goguryeo empire of Korea between 598 and 614, it resulted in the defeat of Sui and its eventual fall in 618.

Background

Sui united China in 589, defeating the Chen Dynasty and ending the division of the continent that spanned almost 300 years. After the unification of China, Sui asserted its position as the overlord of all Asia. Most countries submitted themselves, but Emperor Pyeongwon , and Emperor Yeongyang after succeeding his father in 590 was adamant in maintaining equal relationship with the Chinese dynasty, as they saw themselves as the true rulers in north.

Wendi of Sui, displeased with challenge of Goguryeo, sent the diplomatic papers in 596 that amounted to an ultimatum to the Korean dynasty. Though Yeongyang appeared to comply with the ultimatum, he made his hostility against Sui known in the following year, 597, by launching a preemptive assault against Chinese outposts along the border in present-day Hebei province.

First War of 598

Wendi was outraged with the assault, and ordered his fourth son, Yang Yang, to conquer Goguryeo with an army of 300,000. The invasion failed. The historical records of the time record that the withdrawl was due to pest and rough weather, though further research indicated that constant ambushes by Gouguryeo forces, led by Admiral Kang Yi-sik, were decisive. Up to 90% of the invasion forces were killed in Goguryeo.

Second War of 612

Preparation for Campaign

Yangdi, when he succeeded in 604, built the Grand Canal which connected the north and south of China. Though the public reason for constructing the canal was to improve transport for economic purposes, a main underlying reason was the transportation of troops for massive campaign against foreign countries, especially Goguryeo. During the construction, millions of workers died, which had an adverse impact on support for the Sui regime.

After the construction of the canal, Yangdi ordered the conscription of soldiers in 611 and instructed them to gather in present-day Beijing. The force gathered by the end of the year was one of the greatest in civilisation; 1,138,000 combat troops were mobilised. The support troops, responsible for logistics and transportation of resources, are believed to have dwarfed even that number. The total strength of the army is in dispute, with estimates ranging from 3 to more than 5 million. The army began to depart in early 612, and due to its size, it took 40 days for them all to depart. The long line of soldiers stretched for about 400 kilometers.

Campaign in Manchuria

Yeongyang acted decisively. He gave up the buffer zone that he had acquired after the war of 598, as he thought that the area was not suitable to face such a vast army. The Goguryeo troops retreated behind what is now known as Liao He. One fortunate event for the Koreans was that the river melted much sooner than usual; by the time Yangdi arrived, the ice had all melted. Undaunted, Yangdi ordered the construction of three bridges across the river. The bridges however, were too short to reach the opposite bank, and Goguryeo's ambush 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.

Yangdi ordered his generals before the campaigns not to make individual decisions regarding the movement of troops, but to report to him to receive instruction. This order hampered Sui strategy throughout the campaign. During the campaign against Yodong fortress, the request of surrender at the danger of the fall of the citadel resulted in halt of the assault. The commander had to send a messenger to the emperor for the approval of the surrender; by the time the messenger returned, Goguryeo's troops were again ready to defend the fortress. Exploiting this flaw in the Sui system, Yodong and other citadels of Goguryeo continued to hold on; by June, five months after the beginning of the campaign, not a single fortress fell to Sui. Yangdi felt that a fundamental change in strategy was in order.

Campaign against Pyeongyang

The new strategy was to keep the fortresses of Goguryeo in Manchuria at bay, while sending a contingent of army and navy to take Pyeongyang, Goguryeo's capital. An army contingent, with a strength of 305,000, and a naval fleet of some 200,000 were dispatched.

Naval Assault

The navy reached the bay of the Taedong River before the army. Seeing that the Sui army had not arrived yet, Yeongyang sent a small host to engage the Sui navy in battle. After a short engagement, feigning defeat, Goguryeo troops retreated into the city. The Sui naval commander, Nae Ho Ya , considering this a chance to add to his merit, led a host of some 100,000 to lay assault against Pyeongyang. When the army reached the outer citadel, however, they found the wall empty and the gate open. Upon entering the citadel, they found the valuables and weapons laid out over the place. Being cautious, Nae prohibited the troops from taking the goods. While passing through a large Buddhist temple, Goguryeo troops ambushed Sui troops, but it failed and survivors retreated yet again. Nae, considering the ambush in the temple a decisive move of Goguryeo that failed, allowed the looting of the outer citadel.

While the looting of the citadel was at its peak, special contingent of Goguryeo troops, with strength of a few thousands, ambushed Sui troops. Confused and disoriented, Nae didn't offer resistance but ran to the fleet at the sea, his host reduced to a couple of thousand. Disheartened by his failure, Nae changed his plan to wait for army contingent against Pyeongyang and combine to lay seize on the city.

Sui Army's Campaign

The army contingent, led by generals Wu Joong Mun and Wu Mun Sul , had a problem of their own. While the supply could be transported safely within China, once it reached Goguryeo territory, there was a danger of ambush by Goguryeo forces. Yangdi solved the problem by instructing each soldiers to carry to food for himself, which greatly increased each soldier's burden. Many resorted to discarding the food that by the time they had reached Amrok (Yalu river, the army was in acute shortage of food.

Yeongyang, to ascertain the strength of Sui army, ordered his Commander, Field Marshal Ulji Moonduk, to feign the intention to surrender and negotiate with Sui generals. Generals Wu had their own secret order from Yangdi to capture Yeongyang or Ulji at all cost. However the advisers to them convinced the generals that it is not right to arrest the negotiator from opposition, and they had let Ulji go. Though Wu Joong Mun later regretted his decision and asked Ulji to come back, Ulji, already aware of Wu's intentions and hunger of Sui troops did not comply.

Two generals Wu argued about the course of action to take, and had to consult Yangdi. Yangdi believed Wu Joong Mun's strategy, to chase Goguryeo's army and capture Pyeongyang was right. The Sui troops began the advancement into Korean peninsula. Ulji maneuvered the Goguryeo troops that they faced Sui army seven times a day and retreated every time. When this continued to happen, Wu Joong Mun began to underestimate Ulji. Wu eventually advanced to about 20km from Pyeongyang. However he soon realised that campaign agains Pyeongyang was impossible, as the troops had nothing to eat.

While Wu was in this dillema, neither able to advance or retreat, Ulji sent him a poem, in which he mocked Wu's ability as a commander. When no response came, Ulji sent an emissary, proposing that he will escort Yeongyang to Yangdi to surrender, if the Sui army withdrew. Seizing this excuse to retreat, Wu withdrew his troops.

Great Battle of Salsu and retreat of Yangdi

When Sui army had reached Salsu (thought to be present Chungcheon river ), the water level was shallow. Ulji had already selected Salsu as the decisive turning point of the war, and cut off the flow of water, storing them behind a dam. When Sui army had half crossed the river, Ulji opened the dam,, and onslaught of water left thousands of Sui soldiers dead. Goguryeo host charged the remaining Sui army from the rear, and with command system destroyed, each soldiers had to find their own way. The historical record records that Sui soldiers had to run almost 200 kilometers that day to escape death. The result was one of the momentous victory in Korean military history-from 305,000 army contingent, only 2,700 managed to run back to Manchuria. Hearing about the decimation of army, Navy also retreated back to China. Yangdi, realising that he had no choice, withdrew his troops from Manchuria in defeat.

Third War of 613

Yangdi however had not given up the dream of conquering Goguryeo. He invaded Goguryeo again the following year 613, but had to retreat again without any success. While Yangdi was away, Yang Hyen Gam , a member of Yangdi's inner circle, started rebellion against Yangdi. Fearing the attack from two axis, Yangdi was forced to retreat and use his army to defeat Yang's rebellion.

Fourth War of 614

After crushing the revolt, he invaded Goguryeo again in 614. Though he managed to breach the first line of defence of Goguryeo, he could not advance further, due to shortage of supply and constant distraction by Goguryeo's ambushes. Yeongyang, believing this a chance to end the war, offered the peace, though he did not agree to any of Yangdi's demand, including his surrender and holding Yangdi to be his overlord. Perceiving that he has no chance of success, Yangdi relented, and withdrew his troops from Goguryeo.

Aftermath

Yangdi's continuous campaigns against Goguryeo led millions of people in Sui to death in one way or the other. The discontent against the regime rose, and after the last campaign, the revolts erupted across China. Yangdi moved the capital to south to be out of chaos and to calm things, but the revolts were too widespread to handle. The events culmulated in the fall of Capitals, Xian and Liaoyang to rebels in 617, and murder of Yangdi and fall of Sui in 618.