A more serious challenge descended upon Koguryo during the second half of the sixth century. The four-power set-up in East Asia was collapsing, when the new Sui Empire emerged. Sui managed to unify numerous nations competing for supremacy in the Huang and Yangtze River regions and even destroyed Tuchueh, who had been ruling the northeastern plains. This led Sui forces face to face with Koguryo, which was still pursuing expansionist policies. In preparation for a war with Sui, Koguryo collected lots of information, spurred on weapons development, strengthened its internal cohesion and braced itself for a war. In 612, Sui attacked Koguryo with millions of troops. Koguryo scored a major victory at the battle of Salsu River by annihilating most of the 305,000-strong Sui troops. Subsequently, Koguryo successfully repulsed three more major attacks. Ultimately, this series of defeats led to the downfall of the Sui Empire itself.

Thirty years later, Koguryo had to wage a war against the Tang, the Chinese successor to Sui. In 645, it repulsed hundreds of thousands of Tang forces in combats at the western frontier's strategic points of Sinseong, Geonan and Anshi fortresses and Mt. Jupil. The commander of Chinese forces was King Taejong himself, known as the greatest king in Chinese history. In 661, mobilizing hundreds of thousands of troops, Tang again came attacking Koguryo, but its ambitions were frustrated the following February as the main component of Chinese forces was routed.