Silla's ally became its next opponent

A wartime alliance with different end goals

In the seventh century, the Korean kingdom of Silla faced two rivals: Baekje to its west and Goguryeo to its north. Tang China offered the military force Silla needed to defeat them. The partnership worked: Tang and Silla captured Baekje in 660 and Goguryeo in 668.

But defeating their shared enemies did not settle who would rule the conquered lands. Tang treated victory as an opportunity to extend its own authority, establishing military administrations in former Baekje and Goguryeo territory. Silla, which had fought to strengthen its position on the peninsula, now faced the prospect of a powerful ally becoming its new overlord.

From cooperation to a struggle for control

Silla resisted Tang’s administration and fought Tang forces from 670 onward. This was not simply a sudden change of sides: the alliance had joined two powers with overlapping military aims but different political ambitions. Once the old kingdoms were defeated, that difference became impossible to ignore.

Silla’s forces gradually gained control of much of the peninsula south of the Taedong River. By 676, Tang had withdrawn its forces from the peninsula, and Silla had secured a kingdom far larger than the one it had started with. It did not, however, unite all of the former Goguryeo realm; a new state, Balhae, later rose in the north.

What Silla’s victory actually achieved

The alliance let Silla use Tang’s strength to break the balance among the Korean kingdoms. The war that followed prevented Tang from turning that battlefield success into lasting rule over Silla’s gains. Silla’s achievement was therefore not the conquest of all Korea, but the creation of a dominant southern kingdom that had helped invite an empire in—and then fought to keep that empire from staying.