The Battle of Salsu, led by General Eulji Mundeok in 612 during the reign of King Yeongyang, stands as one of the most dramatic defensive victories in East Asian history. Following its unification of China in 589 AD, the Sui Dynasty relentlessly pressured Goguryeo, viewing it as a defiant state that needed to be brought under its suzerainty. Emperor Yang of Sui, driven by imperial ambition and a desire to solidify his empire's prestige, launched an unprecedented invasion in 612. He mobilized a colossal army of 1.13 million regular troops, with support personnel swelling the total to over 2 million, making it one of the largest armies ever assembled in ancient history. For Goguryeo, this was an existential crisis. The massive Sui army initially struggled to breach Goguryeo's formidable border defenses, particularly at fortresses like Liaodong. Frustrated by the prolonged siege, Emperor Yang ordered a special detachment of 300,000 elite troops, led by Yuwen Shu and Yu Zhongwen, to bypass the fortresses and march directly on Pyeongyang, Goguryeo's capital. General Eulji Mundeok, a brilliant strategist, seized this opportunity. He employed a masterful strategy of feigned retreats and scorched-earth tactics, luring the Sui vanguard deeper into Goguryeo territory. He further confused the Sui commanders and depleted their supplies by sending deceptive messages, pretending to surrender. The exhausted and undersupplied Sui forces eventually reached the vicinity of Pyeongyang but found the city impregnable, with their supply lines dangerously stretched. Eulji Mundeok then sent a subtle, mocking poem to Yuwen Shu, hinting at their predicament and suggesting retreat. As the Sui special detachment began its desperate retreat, Eulji Mundeok launched his decisive attack at the Salsu River (modern Cheongcheon River). Goguryeo forces had secretly dammed the river upstream. As the Sui army was crossing, the dam was breached, unleashing a torrent that drowned a vast number of Sui soldiers. Simultaneously, Goguryeo forces ambushed the remaining Sui troops, inflicting catastrophic casualties. Out of the 300,000-strong special detachment, only a mere 2,700 soldiers reportedly managed to escape back to Sui territory. The Battle of Salsu was a monumental victory for Goguryeo, securing the kingdom's independence and showcasing Eulji Mundeok's unparalleled strategic genius. For the Sui Dynasty, it was a devastating blow, leading to immense human and financial losses. Emperor Yang's subsequent two invasions (613, 614) also failed, further draining Sui resources and popular support. These military failures, combined with internal unrest, directly contributed to the collapse of the Sui Dynasty in 618. The victory at Salsu allowed Goguryeo to remain a dominant power in Northeast Asia for several more decades, cementing its place as one of the most significant defensive victories in Korean history.