The victory at Ansi Fortress in 645 AD, during the reign of King Bojang of Goguryeo, is a pivotal event in East Asian history, marking Goguryeo's successful repulsion of a massive invasion force led by Emperor Taizong of the Tang Dynasty. This battle stands as a symbolic triumph, showcasing Goguryeo's formidable national power and the indomitable will of its people to resist. **Historical Background:** In the early 7th century, following the collapse of the Sui Dynasty, the Tang Dynasty emerged as the new hegemon of East Asia. Emperor Taizong of Tang, learning from the Sui's failed attempts, harbored ambitions to subjugate Goguryeo and eliminate the perceived threat from the east. His pretext for invasion solidified when Yeon Gaesomun of Goguryeo staged a coup, assassinated King Yeongnyu, and enthroned King Bojang, pursuing a hardline policy against Tang. Starting in 644, Tang began extensive preparations for a grand expedition. In 645, Emperor Taizong personally led an immense army, estimated at around 300,000 strong, invading Goguryeo by both land and sea. The Tang forces initially achieved rapid successes, capturing several key fortresses such as Gaimo, Liaodong, and Baekam, advancing swiftly towards Ansi Fortress, a crucial stronghold on the path to Goguryeo's capital, Pyeongyang. **Main Events and Process:** Ansi Fortress was a strategically vital point, connecting the Yalu River defense line to Pyeongyang; further advance without its capture was impossible. Emperor Taizong laid siege to Ansi, launching relentless assaults. However, under the command of its fortress lord (traditionally known as Yang Manchun, though his name is not officially recorded), the people of Ansi staunchly resisted. The Tang army employed various siege engines, including trebuchets and battering rams. They even embarked on a monumental engineering feat: constructing a massive earth mound (土山) taller than the fortress walls, intending to overlook and breach them. This mound, built by hundreds of thousands of soldiers over 60 days, was nearing completion when a section of it suddenly collapsed. The quick-thinking Goguryeo defenders seized this opportunity, occupying the collapsed mound and turning it into part of their defense. This significantly demoralized the Tang forces and allowed Goguryeo to further strengthen its walls. After months of intense fighting, with winter approaching, supply lines stretched thin, and soldiers suffering from fatigue and disease, Emperor Taizong had no choice but to order a retreat. The retreating Tang army suffered immense casualties from Goguryeo's pursuit and the harsh winter conditions. **Historical Significance and Aftermath:** The victory at Ansi Fortress was a testament to Goguryeo's status as a powerful East Asian nation and a decisive turning point that thwarted Emperor Taizong's ambition to conquer Goguryeo. This defeat was a major shock to the Tang Dynasty, incurring massive human and material losses. Although Taizong attempted further smaller-scale invasions of Goguryeo later, he never again launched such a grand expedition. Internally, the victory solidified Yeon Gaesomun's power and instilled a sense of pride and unity among the Goguryeo people. While Goguryeo would eventually fall decades later, the Battle of Ansi Fortress remains a historical legacy symbolizing an indomitable spirit that defended the nation against foreign invasion. To this day, it serves as a source of national pride for Koreans. The fortress lord Yang Manchun, though his name is not found in official historical records, is remembered as a national hero.