[34] The kingdom was originally called Guryeo (Old Korean: 句麗, Yale: Kwulye, /ɡuɾ.jʌ̹/) or something similar to kaukuri (/ko̞ːkɯ̟ᵝɾʲi/)),[35][36] Both words were derived from "忽" (*kuru or *kolo) which meant castle or fortress.

[43] The American historian Christopher Beckwith offers the alternative proposal that the Guguryeo people were first located in or around Liaoxi (western Liaoning and parts of Inner Mongolia) and later migrated eastward, pointing to another account in the Book of Han.

[51][full citation needed] According to Book 37 of the Samguk sagi, Goguryeo originated north of ancient China, then gradually moved east to the side of Taedong River.

[53] The Records of the Three Kingdoms, in the section titled "Accounts of the Eastern Barbarians", implied that Buyeo and the Yemaek people were ethnically related and spoke a similar language.

[citation needed] Book 28 of Samguk sagi stated that "many people of China fled [to] East of the Sea due to the chaos of war by Qin and Han".

A traditional account from the "Annals of Baekje" section in the Samguk sagi says that Soseono was the daughter of Yeon Tabal, a wealthy influential figure in Jolbon[67] and married to Jumong.

[70][71] Goguryeo developed from a league of various Yemaek tribes to an early state and rapidly expanded its power from their original basin of control in the Hun River drainage.

Early expansion might be best explained by ecology; Goguryeo controlled territory in what is currently central and southern Manchuria and northern Korea, which are both very mountainous and lacking in arable land.

From the increase of resources and manpower that these subjugated tribes gave him, Taejodae led Goguryeo in attacking the Han Commanderies of Lelang and Xuantu on the Korean and Liaodong peninsulas, becoming fully independent from them.

Surrounded by these commanderies, who were governed by aggressive warlords, Goguryeo moved to improve relations with the newly created dynasty of Cao Wei in China and sent tribute in 220.

In the early 4th century, the nomadic proto-Mongol Xianbei people occupied northern China;[84] during the winter of 342, the Xianbei of Former Yan, ruled by the Murong clan, attacked and destroyed Goguryeo's capital, Hwando, capturing 50,000 Goguryeo men and women to use as slave labor in addition to taking the Queen Dowager and Queen prisoner,[86] and forced Gogukwon to flee for a while.

[88] Turning to domestic stability and the unification of various conquered tribes, Sosurim proclaimed new laws, embraced Buddhism as the state religion in 372, and established a national educational institute called the Taehak (Korean: 태학; Hanja: 太學).

[89][98] To the west, he destroyed neighboring Khitan tribes and invaded Later Yan, conquering the entire Liaodong Peninsula;[89][98][103] to the north and east, he annexed much of Buyeo and conquered the Sushen, who were Tungusic ancestors of the Jurchens and Manchus;[104] and to the south, he defeated and subjugated Baekje, contributed to the dissolution of Gaya, and vassalized Silla after defending it from a coalition of Baekje, Gaya, and Wa.

[98] Gwanggaeto's exploits were recorded on a huge memorial stele erected by his son Jangsu, located in present-day Ji'an on the border between China and North Korea.

In 551 AD, Baekje and Silla entered into an alliance to attack Goguryeo and conquer the Han River valley, an important strategic area close to the center of the peninsula and a very rich agricultural region.

After Baekje exhausted themselves with a series of costly assaults on Goguryeo fortifications, Silla troops, arriving on the pretense of offering assistance, attacked and took possession of the entire Han River valley in 553.

It effectively made Baekje the weakest player on the Korean Peninsula and gave Silla an important resource and population rich area as a base for expansion.

He proceeded to enthrone Yeongnyu's nephew, Go Jang, as King Bojang while wielding de facto control of Goguryeo himself as the Dae Magniji (대막리지; 大莫離支; a position equivalent to a modern era dual office of prime minister and generalissimo).

[118] Since the early 7th century, Silla had been forced on the defensive by both Baekje and Goguryeo, which had not yet formally allied but had both desired to erode Sillan power in the Han Valley.

[119] Although these invasions were ultimately unsuccessful, in 643, once again under pressure from the Goguryeo–Baekje alliance, Jinpyeong's successor, Queen Seondeok of Silla, requested military aid from Tang.

[120] Ansi City (in modern Haicheng, Liaoning), which was the last fortress that would clear the Liaodong Peninsula of significant defensive works and was promptly put under siege.

[120] However, the capable defense put up by Ansi's commanding general (whose name is controversial but traditionally is believed to be Yang Manchun) stymied Tang forces and, in late fall, with winter fast approaching and his supplies running low, Tang forces under the command Prince Li Daozong attempted to build a rampart to seize the city in a last ditch effort, but was foiled when Goguryeo troops managed to seize control of it.

[134] In the summer of 666, Yeon Gaesomun died of a natural cause and Goguryeo was thrown into chaos and weakened by a succession struggle among his sons and younger brother.

The violent dissension resulting from Yeon Gaesomun's death proved to be the primary reason for the Tang–Silla triumph, thanks to the division, defections, and widespread demoralization it caused.

[136][137] However, there was much resistance to Tang rule (fanned by Silla, which was displeased that Tang did not give it Goguryeo or Baekje's territory), and in 669, following Emperor Gaozong's order, a part of the Goguryeo people were forced to move to the region between the Yangtze River and the Huai River, as well as the regions south of the Qinling Mountains and west of Chang'an, only leaving old and weak inhabitants in the original land.

[166] Scholars and archaeologists posit that contemporary Korean and Japanese adversaries first encountered fortified Goguryeo mounted troops between the fourth and fifth centuries CE.

Two hunts per year, led by the king himself, maneuvers exercises, hunt-maneuvers and parades were conducted to give the Goguryeo soldier a high level of individual training.

With their steep-banked rivers, they lack good fields; and though they plow and till energetically, their efforts are not enough to fill their bellies; their custom is to be sparing of food.

Some ruins are also still visible in present-day China, for example at Wunü Mountain, suspected to be the site of Jolbon fortress, near Huanren in Liaoning province on the present border with North Korea.

Thomas Chase points out that despite the growing online discussion on this subject, this has not led to a more objective treatment of this history, nor a more critical evaluation of its relationship to national identity.