Silla ( 신라 )
Queen Jinseong ( 진성여왕 ) ruled during the time of forced tax collections and peasant rebellions in Silla. She was also the first ruler of Silla during the time period known to historians as the Later Three Kingdoms Period. She abdicated the throne in 897 and died shortly thereafter. King Hyogang ( 효강왕 ) succeeded her to the throne, but was unable to do anything to quell the ongoing turmoil on the Korean peninsula. It is said that he became an alcoholic and completely neglected his duties at the end of his reign. King Gyeongmyeong ( 경명왕 ) then unsuccessfully sought help from Tang and the Silla capital of Gyeongju was eventually sacked by Hubaekje in 927. King Gyeongsun ( 경순왕 ) was placed on the throne after the fall of Gyeongju, but he held no real power and the only notable aspect of his reign is his surrender to Goryeo in 935. Thus, the mighty kingdom of Silla finally came to an end after 993 years of existence.
I really got the sense that this had become a pitiful state by the time of the Later Three Kingdoms Period. There are stories of Queen Jinseong bringing young boys into the palace for her own pleasure, King Hyogang succumbing to alcoholism and it is said that Hubaekje found King Gyeongae ( 경애왕 ) partying when they overtook the capital. It just seems as though Silla had become completely corrupt and helpless during this time period and that the ruling class was either ignorant of or unconcerned with their impending doom.
Despite the sad state of affairs in Late Silla, the Silla Kingdom is generally viewed as the progenitor of Korean culture. Balhae certainly had some contributions, but the ruling class of Balhae had no significant impact in Goryeo's history. In contrast, Goryeo saw itself as the successor to Silla and therefore adopted many cultural aspects of the Silla Kingdom. I think it's hard to overstate the influence of a kingdom that existed on the Korean peninsula for almost 1,000 years.
Hubaekje ( 후백제 )
One of the leaders who arose during the peasant rebellions was named Gyeon Hwon ( 견훤 ). Gyeon Hwon was actually born as a peasant and had successfully risen through the ranks of the Silla military due to his meritorious service. In 892, during the time of widespread turmoil within the Silla Kingdom, Gyeon Hwon established a revival of the Baekje Kingdom at modern-day Gwangju with the objective of avenging the last king of Baekje, King Uija. Gyeon Hwon officially adopted the name Hubaekje, or Later Baekje, in 900 and sought to expand his kingdom. He accomplished this by establishing trade with China and battling with Silla and the other upstart kingdom of Hugoguryeo. He attacked multiple areas controlled by Silla, including the Silla capital in 927 and placed King Gyeongsun on the throne after King Gyeongae committed suicide.
Despite initial success, Gyeon Hwon ultimately brought about his own downfall due to his tyrannical rule. He also attempted to pass power to his fourth son, Geumgang, rather than his eldest son, Singeom. Singeom plotted with two of his brothers to overthrow Gyeon Hwon and was successful in accomplishing this scheme in 935. Gyeon Hwon responded by joining the Goryeo military and leading an attack against his former kingdom, which resulted in the end of Hubaekje in 936. Gyeon Hwon also died that same year.
Hugoguryeo ( 후고구려 ) / Majin ( 마진 ) / Taebong ( 태봉 )
Gung Ye ( 궁예 ) was born into the Silla royal family (it is believed he was the son of either King Heonan or King Gyeongmun), but a prophecy was made that predicted that he would bring an end to the Silla Kingdom. Therefore, the king ordered to have him killed, but his mother conspired with her maid to save the child. When the king's servants came to kill the child, she threw him from a second story window and the servants believed that he had fallen to his death. However, the maid caught the child and raised him in secret.
Gung Ye joined the rebellions that were taking place around the country in 892 and eventually became one of the most powerful rebels. He initiated a rebellion at Songak (modern-day Gaeseong) in 898 and eventually started the Hugoguryeo Kingdom in 901. He changed the name of the kingdom to Majin in 904 and moved the capital. It was from this capital that his kingdom began to rapidly expand and he eventually took over almost all of the land that had once been possessed by Silla.
However, like his counterpart in Hubaekje, Gung Ye was also a despotic ruler and he even went so far as to claim himself to be Maitreya Buddha to rationalize his rule. He again changed the name of his kingdom, this time to Taebong, in 911. Like many despotic rulers, Gung Ye was paranoid that the men around him were plotting against him and as a result he had many of his top officials, including his own family members, executed. This sort of tyrannical rule simply could not last, and he was finally overthrown by his generals in 918 and killed by his own people as he was attempting to flee the kingdom.
Goryeo ( 고려 )
Wang Geon ( 왕건 ) was born into a wealthy merchant family at Songak. Wang Geon joined the rebellions against Silla and eventually joined forces with Gung Ye in his attempt to consolidate control of the rebel forces. Wang Geon proved to be an extremely talented military commander and he was promoted to general in Gung Ye's army. He led many successful military campaigns and it is said that the public liked him for his leadership and generosity.
A quick side note: The source for much of this material is the Samguk Sagi, which was written during Goryeo rule. Although the exact personalities of these individuals is unknown, this might explain why Gung Ye is presented as a merciless tyrant, while Wang Geon seems like a benevolent and strong leader.
The four generals who overthrew Gung Ye decided to place Wang Geon, later to known as Taejo ( 태조 ), on the throne in 918. Taejo changed the name of the state to Goryeo and moved the capital to his hometown of Songak, which provided him with a strong local power base. Taejo decided to form a friendly relationship with Silla and instead focus his aggression against Hubaekje. Taejo even personally led his troops into battle when Hubaekje invaded Silla and it is said that he won the admiration of the Silla people.
With Silla rendered helpless, the battle for the Korean peninsula was now a two man fight between Goryeo and Hubaekje. These two originally appeared to be very evenly matched, but the turning point came at the battle of Gochang in 930 when the fight turned in favor of Goryeo. Goryeo later defeated Hubaekje forces at Unju in 934 and this battle, combined with Hubaekje's internal turmoil, put victory within reach for Goryeo. Silla's King Gyeongsun capitulated to Goryeo in 935 without resistance and as a result married one of Taejo's daughters and was appointed to the position of Inspector General of Gyeongju. Gyeon Hwon then fled Hubaekje in 936 after his sons plotted against him and provided help to Taejo in defeating Hubaekje. The decisive blow was brought against Hubaekje later that year and Singeom, the new leader of Hubaekje, surrendered to Goryeo control.
Balhae had also been conquered in 926 and many members of the aristocracy fled to Goryeo. Thus, the Korean peninsula had finally become truly unified for the first time in history and it would remain unified for over 1,000 years until the division of North and South Korea in 1948. As for Goryeo's part in Korean history, the dynasty would rule until 1392 when the Joseon Dynasty took control of the peninsula. I am planning on dedicating three posts to Goryeo's four and a half centuries of power.