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How did the Tang dynasty and Silla defeat Goguryeo in 668?

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Map of the Korean Peninsula highlighted in orange, showing North Korea in the north and South Korea in the south. The peninsula is bordered by China and Russia to the north, the Yellow Sea to the west, the East Sea (Sea of Japan) to the east, and Japan across the sea to the southeast. A small inset globe at the top left shows the peninsula's location in East Asia. — Korea Map of the Korean Peninsula, indicating northern and southern regions.

The defeat of Goguryeo in 668 was the result of a strategic military alliance between the southern Korean kingdom of Silla and the Chinese Tang dynasty.12

Following the establishment of the Tang dynasty in China, Goguryeo faced persistent military incursions from the west. Simultaneously, Silla had developed a powerful military machine, notably through the organization of the Hwa Rang Do military corps and a realigned military system under King Jinheung.12

Leveraging this military strength, Silla formed an alliance with the Tang dynasty to challenge the other two kingdoms of the peninsula. This coalition successfully subjugated Baekje in 660 before turning its combined forces against Goguryeo in 668. The fall of Goguryeo marked the end of the Three Kingdoms period and initiated a subsequent decade of conflict, during which Silla successfully expelled Tang forces to establish the Unified Silla dynasty.21