Koguryo–Sui , Tang War
The Northeast Project asserts that the invasion of Koguryo by the Sui dynasty was in fact a civil war to suppress a rebellion and unify China. Furthermore, it contends that Koguryo accepted, without refutation, a royal message from Emperor Yang of Sui, Taejong of Tang justifying the invasion. These claims emphasize the justification for the war stating that Koguryo broke a subordinate relationship with the Sui, Tang dynasty.
The Koguryo-Sui, Tang War began as an effort by the Sui, Tang dynasty to conquer Koguryo which proved to be an obstacle in unifying East Asia under Chinese rule. Sui, Tang dynasty attempted to achieve hegemony throughout Asia, but Koguryo's desire to defend its own territory meant that a compromise was impossible. Neighboring forces including Silla, Baekje, Japan, and the Göktürks became involved according to their own self-interests, and an international war eventually developed in the region.
Facts about the Koguryo–Sui, Tang War