Personal opinion of a member of the History Club - What if he traveled to Japan in the early Goryeo Dynasty?

 During the Samhan Unification War, Goryeo mobilized as many as 90,000 troops, of which more than 50,000 were cavalry. It is said that the Goryeo army maintained a strong standing army through the wealth of approximately 1:1 cavalry infantry, exports of ceramics and silk, and brokerage trade between northern China and southern Japan. For example, it was a country that mobilized 170,000 military power and 300,000 Gwangguns during Yun Kwan's expedition to the Byeolmu class. There are many discussions about Gwanggunsa Temple, but I think King Gwangjong picked up soldiers and converted them into a second army because they break through soldiers of the roughly noble class. The 6th place in the 2nd division, the main force of the imperial family, was about 45,000. I understand that this was a military power that could be mobilized whenever the separate imperial family from the local defense forces, Joojin-gun, wanted. And it is said that Goryeo's naval forces operated a ship of approximately thousand ships even if they could not catch them. Even during the Khitan invasion, Goryeo mobilized 200,000 troops, and General Kang Gam-chan led 200,000 troops when he returned home from the 100,000 won army of Sobaeap. In the Battle of Gwiju, Goryeo stopped Khitan's strong mid- to long-term troops with swords, turned the Gyeonggi soldiers backward, intercepted Khitan troops whose breakthrough power was lost with knights, and finally smashed the camp of Khitan soldiers.   Until now, it was the specification of Goryeo, and Japan was in the Heian period at the time, and I understand that the Fujiwara family was in power and manipulating the state. Although the Musa forces are slowly gaining strength, I understand that they are still a private group of aristocratic forces and that the Musa forces themselves were not very large. The emperor's authority is in the era of regent officials who fell to the ground, but I'm not sure about the exact military specifications that Japan can mobilize. However, in the Goryeo Dynasty, Japan will nearly double its population. Maritime power is insignificant because Japan used a policy to close down after the fall of Baekje. What would have happened if Goryeo had decided to eat Japan at this opportunity and mobilized military power in the 10th and 11th centuries until Deokjong, Jeongjong, Munjong, Soonjong, Seonjong, Heonjong, after the invasion of Khitan in Goryeo. In fact, it is an assumption that Goryeo, a commercial country, attempts to expand the country through external expeditions.