Eulji Mundeok (Ul-Ji Moon Dok) was a military leader of early 7th century Goguryeo, one of the Three Kingdoms of Korea, who successfully defended Goguryeo against Sui China. He is often numbered among the greatest heroes in the military history of Korea. Eulji was born in the mid-6th century and died sometime after 618, although the exact date is unknown. Haedong myeongjangjeon "Biographies of famed Korean Generals", written in 18th century, states that he was from the Mountain Seokda (hanja: 石多山 ) near Pyongyang. At the time of his birth, the kingdom of Goguryeo had grown to be a powerful and belligerent state, constantly warring with its neighbours, Chinese states to its north and west, and its fellow Korean kingdoms Silla and Baekje to its southeast and southwest respectively. In 589, the Sui had reunified China for the first time since the fall of the Han dynasty three centuries before. Early on, the Sui launched several large military campaigns against Goguryeo which was unwilling to submit to Sui dominance. Eulji Mundeok was an educated man, skilled in both martial and literary arts. He eventually rose to become the supreme commander of Goguryeo. His first name, Eulji, may actually be a Goguryeo title.

After the founding of the Sui in 589, a precarious peace obtained for several years between the new Chinese dynasty and Goguryeo. In 597, Yeongyang of Goguryeo launched raids across the Liao River, the traditional border. In response, the Sui invaded Goguryeo, but the invasion force was scattered by a typhoon and it failed. In the early 7th century, however, Emperor Yang of Sui learned of secret Goguryeo correspondence with the Eastern Turkic Khaganate. Emperor Yang demanded King Yeongyang come and submit personally to Sui or face an "imperial tour of his territories". When he failed to do so, Emperor Yang prepared for war. He mustered an army of over 1,133,000 troops and more than 2 million auxiliaries and personally led them against Goguryeo in 612. They quickly overran Goguryeo's border defenses, camped on the banks of the Liao and prepared to bridge it.

Eulji Mundeok, commissioned as a Field Marshal, was called upon to assist in the defence of the nation, and prepared his troops to meet the superior Sui forces with a strategy of false retreat, deception and attack. After the Sui forces crossed the Liao River, a small contingent was sent to attack the city of Yodong, but Eulji's forces drove them out. As the rainy season progressed, the Sui forces launched other small probing attacks, but held off from making any large moves before the end of the rainy season. When the rains stopped, Emperor Yang moved his forces to the banks of the Yalu River in northwestern Korea and prepared for a major battle. Fighting only small engagements at times and places of his choosing, Eulji drew the Sui forces further and further from their supply centers. A Sui advance force of over 305,000 men was sent to take the city of Pyongyang. After allowing the force to approach the city, Field Marshal Eulji ambushed it. His forces attacked from all sides, driving the Sui troops back in utter confusion. His troops pursued the retreating army, slaughtering them at will; records claim that only 2,800 men of the massive force returned alive to the main Chinese army. This battle, the Battle of Salsu, came to be known as one of the most glorious military triumphs in Korea's national history. After the battle, winter began to set in and the Sui forces, short on provisions, were forced to return home.

Eulji Mundeok managed to protect Sin Fortress from a Sui invasion force, but he died not long after. The Sui Dynasty was beginning to disintegrate and Emperor Yang decided that he urgently needed to expand his empire in order to regain power, but two more attacks on Goguryeo by Emperor Yang the following spring met with similar disaster, and eventually internal rebellion in China forced the Sui to abandon their desire for Goguryeo. By 618, the relatively short-lived Sui Dynasty was replaced by the Tang Dynasty. Field Marshal Eulji Mundeok's strategy and leadership had protected Goguryeo from the Chinese expansion to the Korean peninsula.

One of the most distinguished military leaders of the Goguryeo period, Eulji's leadership and tactical acumen was the decisive factor in defeating the Sui invasion. Facing numerically superior forces, he developed a strategy that allowed him to secure a decisive victory. Such tactical success was sufficient to earn him a permanent place among Korea's most famous leaders. Kim Bu-sik, the author of the Samguk Sagi, also attributed the victory over Sui to Eulji. In Korea, Eulji Mundeok has been recognized as one of the greatest figures in its national history. During the reign of Hyeonjong in the Goryeo period, a shrine of Eulji Mundeok was built near Pyongyang. In the succeeding Joseon period, he remained just as revered a figure. Yang Seong-ji, a scholar and high-ranking bureaucrat of the Early Joseon, and An Jeong-bok, a Silhak historian of Late Joseon, both thought highly of him. Furthermore, King Sukjong of Joseon ordered the construction of another shrine in honor of Eulji Mundeok in 1680.

At a time when Korea was suffering under the yoke of Japanese Imperialism, a fuller assessment of Eulji commenced with the Korean historian Shin Chaeho (신채호, 申采浩, 1880–1936), who published a biography of Eulji in 1908 and held him out as an example of Korea's traditional nationalist spirit. Eulji Mundeok is still celebrated as a Korean hero. One of the most preeminent Korean scholars of the 20th century, Lee Ki-baik, noted that Eulji's efforts in halting the Sui attempt at conquest stand as one of the earliest examples of Korean attempts to fend off foreign domination.

Today a main thoroughfare in downtown Seoul, Euljiro, is named after Eulji Mundeok. The second highest Military Decoration of South Korea, Field Marshal Lord Eulji's Order of Military Merit, is also named in his honor. Eulji Mundeok's literary work, the Eulji Mundeok Hansi, is one of the oldest surviving poems in Korean literature. One of the biannual Combined Forces Command exercises between South Korea and the United States was called Ulchi-Focus Lens (UFL) in honor of Eulji Mundeok. It has now been renamed Ulchi-Freedom Guardian (UFG). UFG is the world's largest computerized command and control exercises, focusing on how U.S. and South Korean forces would defend against a North Korean attack. Overall, Eulji Mundeok is remembered as a general that saved Goguryeo from the Sui.