Read the passage below and answer these questions:
1. What were the three Korean states in the first half of the first millennium?
2. In which time period did Sejong the Great make the Korean alphabet?

During the first half of the 1st millennium, Korea was divided between three states, Goguryeo, Baekje, and Silla, together known as the Three Kingdoms of Korea. In the second half of the 1st millennium, Silla defeated and conquered Baekje and Goguryeo, leading to the "Unified Silla" period. Meanwhile, Balhae formed in the north, superseding former Goguryeo. Unified Silla eventually collapsed into three separate states due to civil war, ushering in the Later Three Kingdoms. Toward the end of the 1st millennium, Goguryeo was resurrected as Goryeo, which defeated the two other states and unified the Korean Peninsula as a single sovereign state. Around the same time, Balhae collapsed and its last crown prince fled south to Goryeo. Goryeo (also spelled as Koryŏ), whose name developed into the modern exonym "Korea", was a highly cultured state that created the world's first metal movable type in 1234.[3][4][5][6][7][8] However, multiple incursions by the Mongol Empire during the 13th century greatly weakened the nation, which eventually agreed to become a vassal state after decades of fighting. Following military resistance under King Gongmin that ended Mongol political influence in Goryeo, severe political strife followed, and Goryeo eventually fell to a coup led by General Yi Seong-gye, who established Joseon on 17 July 1392. The first 200 years of the Joseon era were marked by relative peace, and during this time the Korean alphabet was created by Sejong the Great and Confucianism became increasingly influential in the kingdom.
1. Goguryeo, Baekje, and Silla were the three Korean states in the first half of the first millennium.
2. Sejong the Great made the Korean alphabet in the first 200 years of the Joseon era.