General Eulji Mundeok
乙支文德      을지문덕
defeated an invasion by China's Sui Dynasty,
Saved Korea's Independence -- in 612
the reason for the name of Seoul's Eulji-ro Avenue
Field Marshal Eulji again employed asymmetrical guerrilla-war tactics, including deception, ambush,
denial of vital supplies (including clean water), superior knowledge of the mountainous terrain and
Korea's harsh weather conditions, to wear-down this massive and turgid Sui Chinese army and navy
in what are now SE Manchurian areas above the Yalu River.  Failing to achieve any victory there, Yangdi
directed one of his generals to invade what is now North Korea using ships going up the Daedong River,
especially trying to take Pyeongyang Castle -- but Eulji frustrated their siege with the same tactics.
The Sui Dynasty invaded and fought against the Goguryeo Kingdom, intending to take all of Korea and
make it a few Chinese provinces, from 598 to 614 CE; the effort and its failure ruined the superpower.
Goguryeo refused to become a tributary state to the Chinese empire, and claimed equal status with it.
In the First Invasion of 598, ordered by founding emperor Wen-di the Great, Sui's 300,000 archer-
cavalry supported by a naval fleet were defeated by rainy summer weather and Goguryeo's successful
asymmetrical guerrilla-war strategy.

2nd (and final) Sui Emperor Yang-di (son of Sui Wen-ti the Great) then sent his army of 1,133,000 men,
perhaps the largest single force in human history, to invade in 612 CE, expecting an easy victory.  It is
recorded that the main ranks of soldiers stretched 300 km long, and took 40 days to pass through the
Great Wall; their supply & maintenance forces must have been even bigger -- and very cumbersome!

Goguryeo King Yeongyang (嬰陽王, r. 590–61, a.k.a.
Pyeong-yang-wang 平陽) appointed General Eulji
乙支 을지 to lead the resistance as Field Marshal, although outnumbered 10-to-1 in solders at the outset.
Eulji then employed his hit-and-run and supply-disruption tactics to lure the weakened main Sui army
south of the Yalu and the Salsu rivers, into what is now North Korea, almost all the way to Pyeongyang
-- poisoning the wells and destroying crops in advance of them.   Sui's top General Yu Zhongwen was
unsure what to do, with so many of his soldiers hungry and falling sick.....
Yu did not immediately accept this wise advice, but continued to lay siege and negotiate.   In a final
clever deception, Eulji persuaded Yu that if the Chinese would give-up on taking Pyeongyang and
retreat back above the Yalu, King Yeongyang would submit to Yangdi as a tributary, and escort Yu's
forces back to Beijing for this.  

While belatedly retreating back across the Salsu River [now named the Cheongcheon River 淸川江i
청천강], Sui's remaining troops were wiped-out by a brilliant ambush.  General Eulji managed to dam
the flow up above a large valley, attacked the Chinese and ran down through that valley, causing Yu's
forces to chase the Koreans, and then broke the dam right when most Chinese were in the valley,
flash-flooding it, then attacking the panicked army, which suffered massive casualties.  

This led to an overall campaign loss of all of the remaining 305,000 Sui soldiers, including General
Yu Zhongwen, except for 2,700 that are recorded to have made it back to the Great Wall.
General Eulji then composed and sent this poem to General Yu Zhongwen, calling for him to retreat.
It is the first solid piece of "literature" we have of all Korean history, and is very famous to Koreans.

The poem reads as follows:

Hangul                Hanja                  English
신책구천문  神策究天文  Your divine plans have plumbed the heavens;
묘산궁지리  妙算窮地理  Your subtle reckoning has spanned the earth;
전승공기고  戰勝功旣高  You win every battle, your military merit is great;
지족원운지  知足願云止  Why then not stop the war and be content…?
Heaven knows how marvelous you are in your strategy,
Earth knows how shrewd you are in your calculation,
Your fame already knows no bounds in this war,
Isn’t it time to know satisfaction in your efforts…?
Goguryeo forces under Eulji, although vastly outnumbered, wiped-out the Sui forces and thus
preserved Korea’s independence from China.   The Battle of Salsu River is listed among the most
lethal "classical formation" battles in world history, and altogether the Korean defeat of Sui's million-
plus troops is one of the most improbable one-sided war-victories that ever happened.

Sui Yang-di invaded Goguryeo again in 613 and 614, out of great frustration, but failed to get beyond
the Liao River in the Liaoning Penninsula, due to Eulji's same strategies.  Mighty Sui finally collapsed
in bankruptcy and revolt in 617, resulting in the birth of the grand Tang Dynasty in 618.
Eulji was posthumously granted the title Mundeok 文德 문덕 or "Cultural Virtue"*, about as high
of an honor as a non-king could have**, and so is known to us as Eulji Mundeok.   Statues and
other tributes to him are common in both North and South Koreas.  The central boulevard of
Seoul, Korea's capital since 1392, was named Eulji-ro by President Park Chung-hee (
later joined
in the 1980s by the three Euljiro metro-subway-train stations of the Green Line; suffix -
ro means a major street)
-- because, besides all the major bank buildings, shopping outlets and ritzy hotels that line its first
few major bocks, in the first alleyway off of it south into Myeong-dong, there stands the Embassy
of China...  "just a little friendly reminder", I suppose...!  

that deok can also be read as morality, ethics, favor, benevolence, kindness, mind, faith or character!
** compare the granting of similar noble title-of-immortality "Munchanghu" to Great Sage "Go-un" Choi Chi-won in 1023.