Yang Liang
As of 611, Sui Dynasty experienced more than two decades of peace and prosperity, after unifying much of China and ending the Chen Dynasty in 589. The Eastern Göktürks, based on the Mongolian plateau, had since become a Sui vassal state under Qimin Khan Ashina Rangan and similarly with Goguryeo, based in the Korean peninsula. There was one brief internal conflict between Emperor Yang of Sui, who became emperor in 604, and his brother Yang Liang the Prince of Han, who had not seen war. When Goguryeo's king Yeongyang refused to pay homage to Emperor Yang in 610, Emperor Yang decided to plan a campaign to conquer it, and both he and the people believed that the conquest would be easy.