Multilateral consideration of the reason why Goguryeo was unable to unify the Three Kingdoms.

 1. A review of the theory that Goguryeo was satisfied with having Baekje and Silla as subordinate countries - Recently, there have been many theories that Goguryeo had a heaven-print system. In fact, Goguryeo used the term "Taewang" for the monarch, which can be said to be an expression of the "Emperor of Goguryeo." This is because Goguryeo, as in King Hwangryong, King Damul, King Donghae, etc., regarded and treated the chief of the conquest area as King, and it is also found in various records that Goguryeo used its own name (not always), Gwanggaeto's inscription, and relics are found in various places. As such, Goguryeo became a hegemony in Northeast Asia and acted so consciously. About countries such as Baekje, Silla, Gaya, Malgal, Kitan, Suksin and others. Therefore, since Goguryeo recognized Baekje and Silla as emperors, the interpretation that they did not conquer them directly even in the heyday of the 5th century when military power was externally released is gaining popularity. It's quite logical, but it's hard to accept it as a major factor. Rather than pretending to be heavenly with Baekje and Silla as their subjects, it would be much more beneficial not only to occupy them and dominate them in terms of strengthening royal power, especially to greatly reduce the burden on the Chinese front and faithfully serve as a rear base. However, Goguryeo did not conquer Baekje and Silla, and made few attempts to occupy it. It is King Jangsu's policy to move south, but Goguryeo did not put more than 30,000 troops into the southern front. It is unusual to help Silla with the dispatch of 50,000 bogeys by King Gwanggaeto Gyeongpyeong An Ho-tae in 400, but since Huyeon's Mo Yong-seong invaded and returned in 399, a year ago, he was able to send troops safely, quickly defeated the Japanese army, withdrew, and mobilized for further invasion. This proves that Goguryeo did not have the ability to launch Invasion from North Korea operations with more than 30,000 troops (according to various records, Goguryeo's troops stationed in Silla did not exceed a hundred units. Rather than leaving it as a combat unit, it would have played a role as a military adviser while monitoring the Silla royal court.) 2. The limitation of Goguryeo's power - When King Balhae ordered his younger brother Daemunye to play black water, the (strategy) party refused. In the past, when Goryeo (Goguryo) was in full swing, it was heroic and firm to fight against the 300,000 military, but once the party disease hit, it collapsed as if it had used the ground.There is a record of [Sindangseo] that it bounced into the shrine while folding it as "Hurak." Based on this, many estimate that Goguryeo's total troops were about 300,000, but during the late Tangtaejong's reign, 150,000 interceptors recruited from 40% of the country's land (south + north), and at least 150,000 to 200,000 troops were conscripted in consideration of the second invasion of the Su Dynasty. Anyway, Goguryeo had to keep almost all of its troops on the fluctuating front, and when the war broke out, at least 100,000 troops were put into the front kiln. Goguryeo handled not only the fluctuating defense line but also the Yalu defense line, so a huge number of troops were deployed on the Chinese front. Therefore, it was too much to organize an invading force that could be mobilized on the southern front. More than 20,000 troops were never mobilized in Goguryeo's Invasion from North Korea operation, except for the 400-year-old King Hotei's Silla Jiwon and King Jangsu's 30,000 troops in 475. Because of the heavy burden of the fluctuating front, Namjin was inevitably unreasonable for Goguryeo. 3. Systematic problems caused by the ancient social system - and the characteristics of the social system of the ancient state should also be noted. Until the Goryeo Dynasty, as well as the three kingdoms of Goguryeo, Baekje, and Silla, the central power was considerably reduced, and the power of local forces was very strong. Even if the central government recruited troops, the military was organized in the form of local families leading and participating in the war, so the interests of local forces were bound by which country they were fighting, and eventually the number of troops was significantly different depending on the nature of the war. In other words, all local tax forces from all over the country were able to unite and fight against the invading forces such as Su and Tang for the purpose of destroying Goguryeo altogether. However, regarding the punitive war, especially Baekje and Silla, which are the goals of Invasion from North Korea, it is inevitable that the central and northern nobles are happy. Even if you fight against Baekje and Silla and take over the territory, you don't know when you will be taken back, and if the damage to your troops is severe during the war, you can't just not participate at all.  In addition, even if it is occupied without any damage and permanent rule is possible, there is no guarantee that it will be assigned as their territory, and if it becomes an emerging nobleman's territory, the existing political situation will be broken, and the king's power will be strong, so the nobles will naturally be reluctant. In addition, even if it becomes his territory, transportation costs will work more and become inefficient. (This is why Goguryeo distributed the war to the noblemen during the Joseon Dynasty.) In the end, Goguryeo could not engage in all-out war due to the inexperience of the ancient social system.The complex factors of my alliance's efficiency and topographical conditions - the Naje alliance has a tremendous synergistic effect in terms of the military aspects of the two countries. For example, in March 481, Goguryeo troops quickly advanced to the vicinity of Gyeongju through a surprise operation, but within 10 to 15 days, a support group from Baekje to Gaya appeared in front of Goguryeo forces, which were on a roll. Considering the speed of dispatch and advancement, it can only be said to be a nearly amazing cooperation system. Eventually, the Goguryeo army retreated and suffered from the chase and lost considerable troops. In addition, the narrow peninsula, mountainous terrain, and rivers were all on the side of Baekje and Silla. Because it was a narrow peninsula, Goguryeo forces could not deal with only one country, had to fight against two countries, and either had to risk complete isolation due to the exposure of rear supply routes, and because it was a mountainous terrain, it would have been difficult to march quickly. In addition, there were many rivers such as Yesunggang River, Imjingang River, and Hangang River, so Goguryeo forces had to carry out several large-scale Doha operations that were very unstable militarily, and even when retreating, they were at high risk of being defeated as if they were naval forces in the Battle of Salsu. In particular, Silla not only had a natural fortress called the Sobaeksanmaek Mountain Range, but also had the upper reaches of the Namhangang River, which was enough to block the rear of Goguryeo troops heading south, and it was much easier to supply and advance ships. In the end, Goguryeo stabbed both countries little by little into a small army of 20,000 to 20,000, but due to the perfect cooperation system and geographical disadvantages of the two countries, it was retreated after taking over the castle, and was almost destroyed without luck. Later, as the Naje alliance broke, Goguryeo eased the burden, but the southern front worsened. This is because the southern front is almost neglected as it is in a hurry to supplement troops on the northern front by conducting an all-out war with the Su and Tang parties. Even in the event of a ceasefire with the Su and Tang parties, they pretended to attack with the intention of making the rear safe during the next war, and the number of troops did not exceed 10,000. There have been many times when Silla mobilized its Hanseong troops to attack Baekje and the territory of Silla near the Goguryeo border was almost empty, but Goguryeo could hardly use its hands, and during the fall of Baekje, it sat down and looked at it with its arms folded. (This is a little extra-discussion, but Yeon Gaemun is responsible for more than half of Goguryeo's collapse so easily by the party's invasion. Liaodongseong Fortress, which had not been shaken for months against the 1 millionth generation of Suyangje, was captured by the army of Dangtaejong in 12 days, and many castles, including Baekamseong Fortress, who fought against the Suyangje army with a determination to fight. At the end of the Daedang War, the floating defense line, which continued to weaken in this way, lost the meaning of the "defense line" and became a "defense point" with only sparse castles left, resulting in the party passing through the fluctuation without major battles and giving Pyongyang easily. If Yeon Gaesomun had excellent political power, he would have stabilized the political situation and returned to an all-out war system, but he split the whole country into two, won the immediate war, caused enormous damage, and miseducated his children. It cannot be denied that Yeon Gaesomun was a great general, but as the head of the state, he has no choice but to evaluate it below the level. It is also questionable whether Bonhae Yeon Gaesomun called for a battle against the Sajangdaedang because he is a patriot. Yeon Gaesomun was steadily building the Great Wall of China, and when King Yeongryu and other Pyongyang aristocrats conspired to kill him, he staged a coup and killed them all. often put forward as a justificationIt's the slogan of Ban Dang. Even if his life was not threatened, it is questionable whether he would have pursued an anti-party policy.)