Goguryeo stood out among the Three Kingdoms of Korea as the most powerful kingdom, more powerful than Silla and Baekje, during the late sixth and early seventh centuries C.E. Sui Dynasty China, succeeding in uniting China in 589 C.E., determined to bring the Korean Peninsula into the Sui Dynasty. Goguryeo, the northernmost of the Three Kingdoms of Korea, had to fall first. Goguryeo was determined to resist the powerful Sui armies rather than enter into a tribute relationship with the Sui Dynasty. Goguryeo insisted on an equal relationship with Sui. To insure that Sui received that message, Goguryeo conducted small scale raiding campaigns on the northern border with Sui.
At that time, Goguryeo's territory extended deep into the territory of present-day China. In appearance, Goguryeo appeared a province of China. Indeed, that is the contention of many Chinese historians today; that Goguryeo constituted a province of China, never a part of Korea. They argue further that Dangun's kingdom, Gojoseon, and Gija's kingdom, Gija Joseon, both had been Chinese provinces and never Korean. At the time of the Sui Dynasty, the emperor looked at Goguryeo as naturally part of the Chinese Empire.
Goguryeo, as mentioned, saw otherwise. Their historians considered Goguryeo the inheritor of Dangun's Gojoseon kingdom, Gija's Gija Joseon, and the Buyeo kingdom, all situated within the territorial sovereignty of Goguryeo. Considering themselves a sovereign kingdom, in league with Silla and Baekje, Gojoseon intended to remain on equal footing with the Sui Dynasty.
The Sui Dynasty launched four massive invasions of Goguryeo between 598 C.E. to 614 C.E. In the first invasion, Goguryeo decimated Sui's 300,000 army and navy, constituting the Emperor Wendi's only defeat in the campaigns of Sui; his son assassinated him in 604. The Sui raised a gigantic army of over one million soldiers, invading Goguryeo a second time in 612. Remarkably, Goguryeo defeated that mighty force. Two more invasions, in 613 and 614, neither of the scale of the second invasion, both ended in defeat for the Sui Dynasty. The enormous suffering brought upon the people of the Sui Dynasty in the four unsuccessful invasions, brought down the mighty Sui dynasty.
Remarkably, as mighty has Goguryeo had been, the kingdom fell to an invasion of combined Silla and Tang China forces in 668 C.E., ending the Three Kingdom Period and inaugurating the era of the North-South States, or Unified Silla dynasty and Balhae dynasty. Balhae formed from the territory in northern Goguryeo while Silla claimed the southern territory of Goguryeo. After the fall of Balhae to the Khitan in the 930s C.E., that northern region never came under the control of Korea again.