Before the Koryo or Goryeo Kingdom unified it, the Korean Peninsula went through a long "Three Kingdoms" period between about 50 BCE and 935 CE. Those warring kingdoms were Baekje (18 BCE to 660 CE), in the southwest of the peninsula; Goguryeo (37 BCE to 668 CE), in the north and central part of the peninsula plus parts of Manchuria; and Silla (57 BCE to 935 CE), in the southeast.
In 918 CE, a new power called Koryo or Goryeo arose in the north under Emperor Taejo. He took the name from the earlier Goguryeo kingdom, although he was not a member of the earlier royal family. "Koryo" would later evolve into the modern name "Korea."
By 936, the Koryo kings had taken on the last Silla and Hubaekje ("late Baekje") rulers, and had united much of the peninsula. It wasn't until 1374, however, that the Koryo kingdom managed to unify almost all of what is now North and South Korea under its rule.
The Koryo period was notable both for its accomplishments and conflicts. Between 993 and 1019, the kingdom fought a series of wars against the Khitan people of Manchuria, expanding Korea northward once more. Although the Koryo and Mongols joined together to fight the Khitans in 1219, by 1231 the Great Khan Ogedei of the Mongol Empire turned and attacked Koryo. Finally, after decades of fierce fighting and high civilian casualties, the Koreans sued for peace with the Mongols in 1258. Koryo even became the jumping-off point for Kublai Khan's armadas when he launched invasions of Japan in 1274 and 1281.
Despite all the turmoil, Koryo made significant advances in art and technology, as well. One of its greatest accomplishments was the Goryeo Tripitaka or Tripitaka Koreana, a collection of the entire Chinese Buddhist canon carved into wood blocks for printing onto paper. The original set of over 80,000 blocks was finished in 1087, but was burned during the 1232 Mongol Invasion of Korea. A second version of the Tripitaka, carved between 1236 and 1251, survives to this day.
The Tripitaka was not the only great printing project of the Koryo period. In 1234, a Korean inventor and Koryo court minister came up with the world's first metal movable type for printing books. Another famous product of the era was intricately carved or incised pottery pieces, usually covered in celadon glaze.
Although Koryo was brilliant culturally, politically it was constantly being undermined by influence and interference from the Yuan Dynasty. In 1392, the Koryo kingdom fell when General Yi Seonggye revolted against King Gongyang. General Yi would go on to found the Joseon Dynasty; just like the founder of Koryo, he took the throne name of Taejo.